A method of acidizing or fracturing subterranean formations comprising injecting into the formation, a composition consisting essentially of a stable acidic gel made by the process of adding from about 25 to about 150 pounds of a linear polyacrylamide having a molecular weight of from about 1 to about 50 million to each 1000 gallons of an aqueous hydrochloric acid solution containing from about 25 to about 35 weight percent of hydrochloric acid. In another method of practicing the invention, the viscosity of the gelled acid is increased prior to injection into the formation by adding to the gel, an aldehyde compound soluble in aqueous solutions. An amount of the aldehyde equivalent to from about 0.01 to about 1.5 weight percent of aldehyde (based on the weight of hydrochloric acid used) is employed. Where this method of practicing the invention is used, the starting acid solution may have a concentration as low as 1 weight percent.