Water plays an important role in in-situ coal gasification. To better understand this role, we conducted a steam tracer test during the later stages of the Hoe Creek No. 3 underground coal gasification field test. Deuterium oxide was used as the tracer. This report describes the tracer test and the analysis of the data obtained. The analysis indicates that at Hoe Creek the injected steam interacts with a large volume of water as it passes through the underground system. We hypothesize that this water is undergoing continual reflux in the underground system, resulting in a tracer response typical of a well-stirred tank.