A process for generation of acid uses a medium comprising a first acid-generating component capable of generating a first acid, and a secondary acid generator, this secondary acid generator being capable of thermal decomposition to form a secondary acid, the thermal decomposition of the secondary acid generator being catalyzed by the first acid. At least part of the medium is exposed to so as to cause formation of the first acid from the first acid-generating component; and the medium is then heated to cause, in the exposed part of the medium, acid-catalyzed thermal decomposition of the secondary acid generator and formation of the secondary acid. The secondary acid generator has a first site bearing a first leaving group and a second site bearing a second leaving group, the first leaving group being capable of protonation by the first acid, with expulsion of the first leaving group to form a cation which electrophilically adds to an unsaturated reagent bearing a proton at the site of addition and a proton-containing nucleophilic grouping at an adjacent site, following which said proton on the reagent is lost and the second leaving group is displaced by said nucleophilic grouping, the second leaving group, in combination with a proton, forming the secondary acid. Preferred variants of the process are of the types described in U.S. Pat. Nos. 5,286,612; 5,334,489; 5,395,736; 5,441,850 and 5,453,345.