Gelled acidic compositions suitable for either matrix-acidizing or fracture-acidizing of subterranean formations, and methods of using said compositions, are provided. Said compositions comprise water, a water-dispersible polymer of acrylamide, an acid, a water-soluble compound of a polyvalent metal wherein the metal can be reduced to a lower polyvalent valence state and cause gelation of the water containing said polymer and said acid, and a reducing agent capable of reducing said metal and causing said gelation.