Stabilization/solidification of hazardous liquids and sludges with portland cement has been investigated as a method of treatment that will bind hazardous materials in a form that minimizes adverse effects on the environment after landfilling. Research has been conducted to determine the long-term acceptability of this technique for disposal of heavy metal sludges by determining the rate of release of metals from solidified/stabilized wastes. The primary concern of this research was to determine the manner in which the heavy metals are released from the paste, as this will be important in extrapolation of the short-term leaching test results to long time periods. These studies utilized upflow leaching columns to determine the effects of acid flux (meq/g/day) and waste particle size on the release of metals from the solidified cement paste containing heavy metal sludges. Two particle size ranges and two acid fluxes were evaluated. Leachates from the column were collected on a daily basis and analyzed for pH, alkalinity, cadmium, chromium, and lead. Release patterns were modelled successfully using a diffusion-based equation. An effective diffusion coefficient and leachability index were calculated for each combination of heavy metal, particle size, and acid flux. These values can be used to predict metal release rates.