The voltage of both CuInSe2and CdTe cells continues to increase after either the onset of illumination or if an illuminated cell is switched from closed to open circuit conditions. The magnitude of the transient voltage effect is typically 10 to 50 mV. Time constants span several decades, 10−4to 10+4s, and are shorter at higher temperatures. The voltage shift is essentially the same at open‐circuit and operating voltages. It depends primarily on the voltage history, not the illumination history, of the cell. The time‐dependent voltage has implications for both the device physics of cells and the interpretation of pulse‐simulator data.