A microwave evapotron is described in which a modified microwave refractometer and a vertical sonic anemometer are combined to measure the vertical transport of water vapor directly by the eddy correlation method. The instrument was used successfully in the field. Ten‐minute samples from this instrument compare favorably with the results from conventional methods. In terms of evaporation, the accuracy is within 0.02 cm/day, and the resolution is within 0.002 cm/day.