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The solubility of plutonium was estimated for waste buried at the Greater Confinement Disposal site in Nevada. The EQ3/6 thermochemical database was modified to include recent data on Pu complex formation, and the solubilities of two critical phases (probertite (CaNaB5O9. 5H(2)O), added as a backfill material; and Ca saccharate) were determined by experiment Reaction path runs were used to model effects of cellulose degradation, including complexation of actinides by organic acids and carbonate, decay of the complexing agents, and the buildup and diffusive loss of CO, through the permeable alluvium. For most waste interaction scenarios, longterm (approximate to 3.10(3) years) concentrations of Pu in pore waters are less than or equal to 10(-7) molal and are dominated by carbonate complexes, although organic complexes could dominate in the first approximate to 10(3) years. In unusual circumstances, carbonation of buried lithium could produce very high Pu solubilities; however, even in such a system, a slight lowering of the effective redox potential dramatically lowers Pu solubility. A sensitivity analysis Suggests that the "base case" calculations are conservative, tending to overestimate long-term solubility, with the system redox and the identity of the organic acids the major sources of uncertainty.