Abstract The optical properties of amorphous self-supporting GeSe1 films show strong non-linear behaviour under the influence of relatively low-intensity c.w. laser irradiation. Discontinuities, bistability and, in special circumstances, oscillation in the optical properties have been found. The importance of thermal effects is experimentally proved. A theoretical model is constructed which explains some of the experimental findings. Evidence is presented, however, that, as well as thermal effects, photostructural changes also play an important role in the bistability and oscillatory phenomena.