It was an unusual gesture, unprecedented, in fact, according to British Prime Minister Tony Blair: The leaders of the world's major industrial nations personally signed the communique issued Friday containing their pledges to double aid to Africa and substantially increase assistance to other poor nations.

The signatures were made at Blair's insistence, because as the prime minister explained, "I wanted to symbolize the strength of our commitment."

But they were also a reminder of the weakness underlying the grand promises made at this past week's summit to demonstrate more generosity toward the world's poor. The amounts actually spent have a history of falling far short of the amounts pledged. That is especially true when -- as in this case -- the 2010 target for the aid increase comes well after those making the pledges are likely to leave office.

That problem was one of the main reasons why many aid experts tempered their enthusiasm for the plan outlined by the Group of Eight nations, despite the size of the $50 billion in additional aid that was envisioned to begin flowing annually five years from now, with half of the increase going to Africa.

"This represents probably the most significant change in attitude by the G-8 toward Africa since the end of the Cold War," said Steven Radelet, a scholar at the Center for Global Development in Washington. "But there is a big difference between a proposal to increase aid and actually doing it."

Kumi Naidoo of South Africa, chairman of the Global Call to Action Against Poverty, agreed. "One of the biggest problems -- not just with the G-8, but other bodies -- is there's a huge compliance deficit," Naidoo said.

The lofty goals of the compact between rich and poor nations were forged at the 2002 summit in Monterrey, Mexico, attended by President Bush and dozens of other world leaders. At that summit, as at this past week's G-8's gathering in the Scottish resort of Gleneagles, there was much rhetoric about how the new pledges of billions of dollars in aid would not mean the end of poverty but rather the beginning of the end. And at Monterrey, leaders had cautioned that the ultimate responsibility would rest with the poor nations to use the largesse wisely and honestly, just as Blair and other speakers cautioned Friday.

Bush's aid initiative at the 2002 summit was greeted with enthusiasm. To the applause of antipoverty campaigners, including the rock star Bono, the president launched the Millennium Challenge Account, a new type of aid aimed exclusively at countries deemed to have sound policies and good governance. Projected spending was $1.7 billion in 2004, $3.3 billion in 2005 and $5 billion per year thereafter.

But the White House did not establish the agency to run the program until last year, and the agency has so far struck agreements with only three countries, the largest being a deal with Honduras funded with $215 million over five years. Adding to the program's woes, its director resigned last month amid criticism that the money was trickling out in much lower quantities than anticipated.

It was not a surprise, therefore, that Bono was admonishing the masses of people inspired by the Live 8 concerts and other celebrity-studded events for Africa that their campaign would have to keep up the pressure long after Gleneagles.

The G-8's communique included country-by-country commitments, most of them consisting of announcements made in the past few weeks in anticipation of the spotlight that would be put on Africa at the summit. But those commitments are based on projected contributions too far in the future to count on.

The pledge of increased European assistance relies mainly on the announcement in May by European Union nations that they would ramp up donations to the equivalent of 0.7 percent of their gross domestic product by 2015, with an interim target of 0.56 percent of GDP by 2010. But the German and Italian governments are in deep fiscal trouble, and their officials have already warned that their aid spending will depend on the health of their economies.

The U.S. pledge is based in part on the assumption that the Millennium Challenge program will reach its target of $5 billion per year, with other spending coming from projected boosts in outlays for a 2003 HIV/AIDS initiative and a recently announced plan to combat malaria. All of that is subject to the annual appropriations process in Congress.

Some experts, however, contend that it is hard to produce a sensible alternative, given the problems with pouring too much aid too quickly into poor countries.

That point was raised Friday by Bob Geldof, the impresario of the Live 8 concerts, who was a member of the Africa Commission established by Blair last year to study the continent's needs. The panel concluded that aid to Africa should be gradually increased, to $50 billion by 2010, based partly on the arguments of African officials, who "told us, 'We can't absorb more, because our governance structures are so weak that we can't take more,' " Geldof said.

Still, the pitfalls involved in summit declarations were vividly on display the day the G-8 meetings ended, when the airy pronouncements from Gleneagles clashed with grim words from top officials of the World Trade Organization over farm subsidies.

The G-8 called for global trade negotiations to eliminate the billions of dollars paid to subsidize crop exports from rich nations. The resulting lower prices put farmers from poor nations at a competitive disadvantage.

But at WTO headquarters in Geneva, officials expressed concern about intransigence among member countries -- many of them the same countries represented in the G-8 -- on negotiating an end to farm subsidies. "I am afraid we have to face the facts," WTO Director-General Supachai Panitchpakdi told negotiators. "These negotiations are in trouble."

Leaders of the world's industrialized nations pledged to double aid to Africa, although the aid would not begin until after they will likely leave office.