The photochemical decomposition mechanism of sulfonium compounds, an important photoacid generator (PAG) in lithography, was extensively studied by ab initio quantum chemical research. The homolytic bond cleavage takes place in the lowest triplet state T-1 as well as the lowest excited singlet state S-1 at the initial stage of the decomposition reaction, but the final products from S-1 are those generated from the heterolytic bond cleavage. The homolytic reaction is maintained from the initial stage to the final products only in the lowest triplet state T-1. In the ground state S-0, the heterolytic bond cleavage occurs at the initial stage, but the final products are those from the homolytic bond cleavage. SDTQ-CI calculations revealed that this mysterious phenomenon is explained as the results generated from the mixing of the lowest excited singlet state S-1 and the ground state S-0. It was concluded that homolytic bond cleavage is the predominant mechanism in the resist films of lithography. The theoretically obtained results were compared with experiment.