Abstract The calibration of a commercially available infra-red surface temperature transducer is described. The instrument uses (i) a thermistor to measure the internal detector temperature and (ii) a thermopile to produce an output voltage, which depends on the temperature of the target surface and the internal detector temperature. Calibration results for the thermistor are shown to be in excellent agreement with the manufacturer's data. Three methods were used for calibration of the thermopile. The calibration results indicate that a single relationship between output voltage and the linear difference between target surface and internal detector temperatures, as proposed by the manufacturer, will lead to significant errors in measured target surface temperature. Theoretical considerations suggest expressions based on power functions of target and detector temperatures. The paper shows that such expressions result in significantly reduced errors, especially when the instrument is used over a wide range of detector temperatures.