A conventional alpha proportional counter which uses air as the counter gas is sensitive to high relative humidity and generates spurious pulses that cannot be distinguished from actual alpha pulses. It has been found possible to operate such a counter satisfactorily at high relative humidity by passing a small current (∼15 mA) through the center wire. In this manner the center wire is heated and the relative humidity of the surrounding sheath of air is reduced sufficiently so that operation of the counter at high relative humidity is comparable to operation with dry air. Two different mechanisms are proposed for the formation of spurious pulses in such a counter at high relative humidity.