Photolysis of triphenylsulfonium salts in solution or polymer films gives 2-, Sand 4-phenylthiobiphenyl isomers by an in-cage reaction, whereas diphenylsulfide and other aromatic photoproducts are formed by cage-escape reactions with solvent. The cage and escape reactions both generate acid, which is the primary initiator for many chemically amplified resists. Photo-CIDNP has been used to characterize the cage and escape reactivity of radical intermediates formed from photolysis of triphenylsulfonium salts. In addition nanosecond laser flash photolysis studies have found key intermediates in the direct and triplet sensitized photolysis of these salts. The photophysics of a number of aromatic polymers was examined to understand how the polymer participates in the photoinitiation process. The polymer fluorescence was quenched by sulfonium salts in solution by a dynamic mechanism, whereas in polymer films the quenching was by a static mechanism. Fluorescence lifetimes for the polymers, estimated from the quenching plots in solution, were relatively short, 4-8 nsec and the values agreed well with those obtained by time-resolved spectroscopy. © 1992, The Society of Photopolymer Science and Technology(SPST). All rights reserved.