The stability of metal-insulator-semiconductor (MIS) solar cells against prolonged illumination was studied. The effects of photon-induced degradation on the current-voltage (I-V) and capacitance-voltage (C-V) characteristics of MIS solar cells were investigated. The results show that all of the solar cells' output parameters were degraded. The photo-degradation mechanism was attributed to an increase in the interface states at the insulator/semiconductor interface.