Denise Stoner works out in Oakton with her iPhone. Marketers are interested in moms like her who often use their smartphone apps. (Matt McClain /FOR THE WASHINGTON POST)

Companies looking to make money on the Web are trying to understand a fast-growing and big-spending demographic: moms.

From Procter & Gamble to AT&T and Pandora, firms are taking notice that mothers, newly armed with smartphones, are becoming a new kind of shopping force online. A decade ago, these women were single and childless 18- to 34-year-olds who captured the hearts of Madison Avenue marketing executives with their voracious consumer appetites. Now, they are older and often in charge of the household wallet.

Moms are the fastest-growing buyers of iPhones, and they are tuning in more frequently and for longer periods than any other group on media Web sites such as Pandora, a streaming music service. Nielsen Research says mothers are far more likely to share photos and news stories on Facebook via smartphones and computers than anyone else.

“We’ve known about the opportunity of online moms for a while now, but then mobile technology came along and blew everything up,” said Marshal Cohen, chief retail analyst at research firm NPD Group.

Between business meetings, in carpool lines and at sports practices, moms are spending downtime on smartphones to update the family calendar, buy soccer cleats, research cheap flights and fit in a few rounds of Angry Birds.

Nielsen calls these women“power moms.” They represent one in five online users — a proportion that is growing quickly — and some research shows they are an even greater force on mobile devices.

The number of moms with smartphones is about equal to men of the same age, but they are adopting the technology at a faster pace. The number of moms who purchased iPhones grew 132 percent in the first quarter of 2011 compared with the same time last year — outpacing men, who rose by 121 percent, according to NPD. Overall, adult purchases of the smartphone grew 117 percent.

Women with children are also among the biggest spenders on the Web — either buying directly from mobile apps or researching products that they buy later.

“Early adopters are no longer young people and men,” said Candace Corlett, president of marketing consulting group WSL Strategic Retail in New York. “What the smartphone does is allow women who are hungry for information to get what they want from the Internet instead of calling up a sister or friend for advice.”

Take Denise Stoner, 48, who on a recent morning pulled out her hot-pink-encased iPhone at least a dozen times within a couple hours. The Vienna resident sent texts to her husband about plans for the day, listened to the Jimmy Buffett channel on Pandora on the way to Tysons Corner and made a few moves on a multiplayer Scrabble app against her sister, who lives in Florida.

As Stoner waited for her 11th-grade daughter to finish her hair appointment, she fired up Netflix. But before she watched her streaming video, she updated friends on Facebook about an annoying run-in with a security guard who gave her a hard time about putting her manicured bare feet up on a mall couch.

“It was just annoying enough to share,” she said.

Indeed, the always-connected and -sharing ethos of moms is what marketers hope to better understand. In the parlance of marketing, these are the most “engaged” users and, in many ways, the most valuable consumers online.

Online coupon site Living Social launched its “Family Deals” division last year because it found so many of its kids-oriented deals outsold other types of offers. In Raleigh, N.C., a deal for Monkey Joe’s bounce house indoor playground sold 3,172 coupons within a few hours. Disclaimer: Living Social’s chief executive, Tim O’Shaughnessy, is the son-in-law of Washington Post Co. Chairman Donald E. Graham.

“We know mom is CEO of the house and maybe CEO or COO of a company, too, and putting 40 to 60 hours a day at their paid job and their unpaid job at home,” said Maire Griffin, a spokeswoman for Living Social. “We need to make things easier for her and help her find the efficiency in her life she’s searching for.”

Firms are dissecting the habits of mothers in hopes of increasing their chances of turning ads into purchases. Web firms are collecting personal information about moms, including what times of the day they’re logged on, if they are connected from home or on the road, and how often browsing turns into a purchase. Firms such as Procter & Gamble, Walt Disney, Comcast and AT&T, which recently commissioned a study on online moms, want to use that kind of data to tailor ads to that demographic.

Pandora is also focusing on those kinds of details. The streaming radio site stumbled upon its huge mom following last year, when it was flooded with queries for children’s musical groups such as “The Sippy Cups.”

A Pandora survey found that the average mom spent twice as much time on the site — and mobile app — than the average listener. About seven out of 10 of those mothers tune in every day.

Pandora is now sharing that information with its advertisers, said Heidi Browning, senior vice president of strategic solutions at Pandora. “This is the greatest gift ever to marketers,” she said.

And as moms pull each other into the smartphone fray, retailers see their opportunities growing.

Teverra Fernandez wouldn’t have bought her Sanyo Zio smartphone this year without the urging of her sister and mom friends. When her son turned 18 months old, friends told her to “get with the program,” the District resident said.

That meant mining smartphone apps for entertainment and learning tools for her toddler. One of her favorites is ToddlePhone Lite, an app that teaches colors and numbers that a friend helped her download onto her Android phone.

“They give me time to take a break,” she said enthusiastically while her son, Malachi, scooped up sand at Friendship Park in the District. When Fernandez, 25, started the app, its familiar music drew Malachi’s attention, and he came running toward the phone.

“A lifesaver,” she joked.