Carboxylic acid groups formed at the surface of poly(4-((trimethylsilyl)methyl)styrene) films upon irradiation with KrF laser pulses at various intensities were analyzed quantitatively. The yield of the acid groups per pulse [acid groups cm(-2) pulse(-1)] as a function of the laser intensity (I, mJ cm(-2) pulse(-1)) was shown to follow a nonlinear relationship; [acid groups cm(-2) pulse(-1)] = Al - BI2 (A and B are constant). The second term, an apparent two-photon process, was attributed to an effect of a coupling reaction of the radical intermediates.