Results of an experimental study of a counter-rotating turbine operating on a pneumatic wave energy conversion system are presented. The vertical capture chamber of the water column of the system has a down-facing water intake. An electrical generator (d-c) is coupled to the turbine by a ten-gear-ratio drive train. Tests of the pneumatic wave energy conversion system were conducted with the mouth at two depths in order to vary the natural oscillating frequency of the water column. The maximum electrical power output of the system was approximately 90 W in the monochromatic waves. This value corresponds to a 36-percent efficiency based on the incident wave power in a wave crest width equal to the diameter of the capture chamber. The theoretical maximum efficiency of such a system has been shown to be 50 percent by several investigators.