The design and performance of a reversing temperature difference measurement system are reported. This system employs five-junction copper-constantan thermopiles for the measurement of δT and δTw, while a linearized thermistor is used to measure Tw. Field performance has been checked against a precision lysimeter as well as against a second temperature difference measurement system in which diodes are used for temperature measurement. In both cases, the agreement between the systems is satisfactory for the measurement of hourly values of the Bowen ratio.