Structural changes were induced in a polyurethane elastomer by irradiating the film with UV light and the effects were monitored through birefringence measurements, thermal analysis and infrared spectroscopy. UV irradiation induces two competing effects: i) weakening and breaking of hydrogen bonding between the urethane groups, which improve the mobility of the hard segments and cause the glass transition temperature to decrease; ii) reticulation in the polymer network causing the glass transition temperature to increase. For short periods of irradiation, up to four hours, breaking of H-bonds predominates and the birefringence increases. Upon further, prolonged irradiation, the effect from reticulation takes over and the birefringence decreases.