Photostability of organic compounds with different emission and photochemical properties has been studied under conditions of nonlinear excitation as a function of the XeCl-laser emission parameters (intensity and degree of polarization) and the character (spontaneous or stimulated) of secondary emission of the studied compounds. Quantum yields of phototransformations are determined, the pulse-average population of the excited state is estimated from the transmittance measurements, and causes of the change in the efficiency of phototransformation are discussed.