Study was made of the cis --> trans isomerization kinetics of a series of azo compounds in polymethyl methacrylate. It was shown that under ultraviolet irradiation a quantity of cis molecules is formed in the stressed states. The stressed cis molecules' relaxation to equilibrium state takes place at temperatures that are far lower than the glass transition temperature. The influence of the relaxation process on the reverse conversion of cis molecules to the trans molecules was investigated along with the influence of temperature.