The two major functions of a backfill are considered to be the exclusion of water from the waste package and chemical retardation of radio-nuclide migration. Preliminary experiments were conducted with Umtanum basalt, water, and various waste forms including simulated spent fuel, supercalcine, and borosilicate glass to determine the feasibility of using basalt as a potential backfill component. Basalt is being evaluated principally as a chemical barrier. Experiments done in the temperature range of 100 to 300°C with only waste form and water show nearly complete dissolution of cesium, rubidium, and molybdenum. However, in parallel experiments, where basalt was added to the system, cesium, rubidium, strontium, and molybdenum were almost completely removed from solution. Analyses of solid run products indicate the formation of pollucite [(Cs,Rb,Na)AlSi2O6·H2O] and powellite (CaMoO4). It is suggested that the glass component of the basalt reacts to form the stable solid phases that fix these elements. Also, rubidium is likely to follow cesium into a plagioclase structure and strontium may be found in plagioclase or powellite. Thus, given the ready availability and reactive nature of basalt, it is a recommended candidate backfill material for use at the Hanford Site.