Self-diffraction of two coherent light waves that record a holographic grating in thin films of azobenzene-containing molecular glass-forming materials is used for intensity enhancement of a weak beam in expense of the strong beam intensity. The intensity of a weak probe beam is doubled at interaction length of only 8 µm thus pointing to a very high specific coupling constant. The characteristic time of the intensity redistribution is relatively long, on the order of 200 s. Consequently, the underlying process of pronounced Kerr-type optical nonlinearity is related, most probably, to the photoinduced mass transfer.