Groundwater sampling and analysis have been carried out at the site of an underground coal gasification experiment conducted by the Lawrence Livermore Laboratory near Gillette, Wyoming. Water from more than 12 wells in the vicinity of the experiment has been sampled before, during, and up to 10 months following gasification. Water samples were analyzed for a variety of characteristics, including the presence of 70 inorganic compounds and elements, phenolic materials, and volatile and semi-volatile organics. We find a greatly increased concentration of organic materials, particularly phenols, just outside the burn boundary and a variety of inorganic species issuing from within the residual ash bed. All detectable contaminants rapidly decreased in concentration with distance, although above-background levels of some species were detected 100 ft from the burn zone. Several important contaminants showed a large decrease in concentration with time due, probably, to absorption. From these data a simple physical picture is generated of the time-development of groundwater pollution near an underground coal gasifier.