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Abstract This paper investigates the effects of temperature on an InGaP (GaInP) 55Fe X-ray photovoltaic cell prototype for a radioisotope microbattery (also called a nuclear microbattery). An In0.5Ga0.5P p-i-n (5 μm i-layer) mesa photodiode was illuminated by a standard 206 MBq 55Fe radioisotope X-ray source and characterised over the temperature range −20 °C to 100 °C. The electrical power output of the device reached its maximum value of 1.5 pW at a temperature of −20 °C. An open circuit voltage and a short circuit current of 0.82 V and 2.5 pA, respectively, were obtained at −20 °C. While the electrical power output and the open circuit voltage decreased with increasing temperature, an almost flat trend was found for the short circuit current. The cell conversion efficiency decreased from 2.1% at −20 °C to 0.7% at 100 °C.