An acid-generating fluid comprising a thermally activated strong acid precursor is provided in the method of the present invention. The thermally activated strong acid precursor comprises sulfur dioxide or a first precursor of a compound adapted to react to liberate sulfur dioxide, and an aldehyde and/or a ketone or a second precursor of a compound adapted to react to liberate an aldehyde and/or ketone. The acid-generating fluid is placed into a subterranean formation, where the first precursor and/or the second precursor are thermally activated, thereby liberating sulfur dioxide and the aldehyde and/or ketone. The sulfur dioxide and the aldehyde and/or ketone are reacted to form a thermally activated strong acid.