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The mechanisms of heavy metal stabilization of calcium-based dry scrubber residue using soluble PO4 3- were investigated. This stabilization technology is presently used in the U.S. and Japan to reduce metals leaching from municipal solid waste combustion residues. At an experimental dose of 1.2 mol of H3PO4/kg of residue and using a relatively dry mixing system, the reduction in the operationally defined fraction available for leaching (using the Dutch Total Availability Test) is 38% for Cd, 58% for Cu, 99% for Pb, and 28% for Zn. pH-dependent leaching (pH 4, 6, 8) showed that the treatment was able to reduce equilibrium concentrations by 0.5−2 log units for many of these metals, particularly Pb and Zn. Depth profiling of particles using secondary ion mass spectroscopy suggests that stabilization is by precipitation of metal phosphate reaction products rather than by adsorption of metals to phosphate particle surfaces. Bulk and surface spectroscopies show that the insoluble reaction products are nanometer-sized, crystalline and amorphous calcium phosphates, tertiary metal phosphates, and apatite family minerals. The geochemical thermodynamic equilibrium model MINTEQA2 was modified to include both extensive phosphate minerals and simple ideal solid solutions for modeling pH-dependent solid phase control of leaching. Both apatite family and tertiary metal phosphate end members and ideal solid solutions act as controlling solids for Ca2+, Zn2+, Pb2+, Cu2+, and Cd2+. The prevalence of small, nanometer-sized reaction products suggest that Ostwald ripening and precipitate maturation has not completely occurred during initial mixing. Nevertheless, soluble phosphate is an effective stabilization agent for divalent heavy metals in waste materials such as scrubber residues.