The aim of this paper is to study the mineralogical transformations of a MSWI fly ash during an industrial stabilization process. This is divided into three successive steps: washing, phosphation and calcination of the fly ash. Special attention was paid to the characterization of the neoformed calcium phosphates in this study. Microprobe analyses carried out on the treated fly ash showed that the whitlockite and hydroxylapatite formed during the process were rich in silicon and aluminium. One of the major minerals contained in the raw fly ash was gehlenite. In order to study the behaviour of this specific phase during the process, the raw fly ash was washed (first with Na2SO4, then with an acidic solution) to concentrate the gehlenite. The washing was successful and the residue obtained was rich in gehlenite and poor in calcite. Mineralogical characterization of the gehlenite showed that this mineral had the same peaks of diffraction as conventional gehlenite but its composition was relatively different from that of the reference mineral. The behaviour of this mineral with phosphoric acid was studied. The results showed that the gehlenite-like phase reacted with phosphoric acid to form brushite, which transformed into whitlockite during the calcination of the phosphated residue. Both brushite and whitlockite contained significant amounts of silicon and aluminium in their lattice, which confirms the analyses carried out on the calcium phosphates contained in the treated fly ash.