The mechanisms of heavy metal stabilization from mineral residues were investigated. The reaction of phosphoric acid with municipal waste incinerator fly ashes and some of its major constituents was assessed. The reaction was monitored by analysis of soluble phosphate contents as a function of time, as well as by pH and temperature variations. Evolution of the solids was followed by X-ray diffraction. Various phosphoric acid concentrations were used and yielded different end products for the case of little and limestone. Silica and calcium sulphate were found to remain inert, while alumina consumed part of the soluble phosphate. Melilite showed it complex process of dissolution and precipitation of amorphous aluminium phosphates. These results help on understanding the phosphate reaction used to stabilize a mineral matrix like fly ash and demonstrate that insoluble minerals are formed such as calcium phosphates which may effectively trap heavy metal ions.