The results of degenerate four-wave mixing studies performed on evaporated films of metal-free phthalocyanine and Langmuir–Blodgett films of a silicon phthalocyanine using subpicosecond 602 nm pulses are reported. The third-order nonlinearity of all the samples is high permitting the observation of the degenerate four-wave mixing signal from even a monolayer. In both cases, the third-order nonlinearity has a resonant character and the observed time-resolved phase conjugate signals provide information on the dynamics of excitons. Simultaneously monitoring first- and second-order diffractions from the transient gratings formed in our samples allowed us to obtain more-detailed information on the factors influencing the temporal behavior of the nonlinear response. It is concluded that the dynamics of excitons determining the nonlinearity is mainly governed by the presence of bimolecular exciton–exciton interaction. Saturation behavior is observed at high light intensities.