Photolysis of triphenylsulfonium salts in solution or polymer films gives 2-, 3- and 4-phenylthiobiphenyl isomers by an in-cage fragmentation-recombination mechanism, and the cage-escape sulfide product diphenylsulfide. Both the in-cage and cage-escape reactions result in acid production. Formation of the in-cage products is favored in viscous solutions, polymer films and in the solid state photolysis of triphenylsulfonium salts. However sensitization by photo-induced electron transfer, or triplet energy transfer, gives only the cage-escape sulfide product via the triphenylsulfur radical, or triplet phenyl radical - diphenylsulfinyl radical cation pair, respectively. In-cage and cage-escape products are also found from photolysis of diphenylhalonium salts. The importance of'cage versus escape' reactivity of onium salts in acid production and in the mechanistic interpretation of their photochemistry, in solution and polymer films, is discussed. © 1991, The Society of Photopolymer Science and Technology(SPST). All rights reserved.