A light-induced expansion of thin films of polymers containing azobenzene chromophores was characterized in real time by single wavelength ellipsometry. An initial expansion of the azo polymer films was found to be irreversible with an extent of relative expansion observed of 1.5-4% in films of thickness ranging from 250 to 1400 A. A subsequent and reversible expansion was observed with repeated irradiation cycles, achieving a relative extent of expansion of 0.6-1.6%. Control over the extent of this reversible expansion was achieved by varying the pump beam power or irradiation duration, allowing these polymers to function as reversible photomechanical materials.