Gladys Knight has been a commercial pop singer for so long that it's easy to forget what a special talent she is. Her show at Constitution Hall last night was a welcome reminder. She sang conventional soul tunes but used them only as the springboard to her improvised vocal flights. These improvisations were the real show.

She sang the verses to "Midnight Train to Georgia" with shuddering conviction. When the verses were done, she really took off. She reached back into her roots in the Mount Mariah Baptist Church in Atlanta. She shouted impulsively, flinging notes around and above the chorus chant. Her extra flourishes expanded this and the other up-tempo songs into new dimensions.

On the slow songs, she extended certain vowels and savored them till they took on a life of their own. On "I Don't Wanna Do Wrong," her wordless moans said more about the conflict between temptation and loyalty than any of the lyrics. She was warmly backed by the Pips, her brother and two cousins who have been her professional partners for 27 years.

Gene Chandler's band, the opening act, didn't show up for last night's concert. Gladys Knight & the Pips return to Constitution Hall -- with Gene Chandler -- for two shows tonight.