We have monitored groundwater quality changes in approximately 60 wells constructed near the 3 underground coal gasification (UCG) experiments carried out at our Hoe Creek site in northeastern Wyoming. A broad range of residual gasification products are introduced into the groundwater system as a result of the UCG process. These groundwater contaminants may be of environmental significance if they find their way, in sufficient concentrations, into surface waters or aquifers from which water is extracted for drinking or agricultural purposes. In seeking to identify effective control technologies or mitigation measures, we have found it important to investigate the mechanisms responsible for the formation and dispersal of the contaminants. For example, an important mechanism for the production of organic contaminants may be pyrolysis reactions along the surfaces of cracks through which hot product gases escape from the cavity during gasification. As gasification continues, these pyrolysis products will be distilled further out in the surrounding coal and deposited on the coal surfaces. Other mechanisms that are believed to be important are post-burn pyrolysis as a result of residual cavity heat and the liberation of minerals from the coal by the action of dissolved CO/sub 2/.