Degradation phenomena under high-voltage stress, referred to as potential-induced degradation (PID) in general, were studied in superstrate-type thin-film photovoltaic (PV) modules, such as CdTe and thin-film Si PV modules. Both deterioration in the PV performance and delamination of the transparent conducting oxide (TCO) layer on the glass substrate were observed by the PID test for both PV modules, although a change in the PV parameters during the PID test with negative voltage application was somewhat different between them. It was also found for both PV modules that recovery from PID is accomplished by positive voltage application and that quick and drastic deterioration in all the PV parameters occurs by the second negative voltage application after recovery. It is suggested that the common origin of PID for superstrate-type thin-film PV modules is damage and delamination of the TCO layer.