Samples of the lazulite-scorzalite, MgAl2(PO4)2(OH)2 — FeAl2(PO4)2(OH)2 solid-solution were synthesized hydro-thermally at 500°C and 0.3 GPa in compositional steps of 0.125 inxFe = Fe/(Fe + Mg) and an additional sample withxFe = 0.0625. The oxygen fugacity was controlled using the QFI-buffer. Under these conditions the solid-solution shows total miscibility verified by X-ray powder diffraction, infrared spectroscopy in the OH stretching region, and 57Fe-Mössbauer spectroscopy. The linear dependence of the molar volume from composition indicates an ideal behaviour of the volumetric properties. The compositional dependence of the lattice parameters a0, b0, and β indicates two different mechanisms for the Fe-incorporation below and above xFe = 0.125. The Fe3+ content determined by 57Fe-Mössbauer spectroscopy increases significantly with decreasing xFe of the solid solutions, from about 5 % of the total iron content to approximately 13.8 %, and was interpreted quantitatively on the basis of an oxidative dehydrogenation reaction. The results indicate a change in the substitution mechanism from an isomorphic substitution which is dominated by simple Mg ⇔ Fe2+ exchange above xFe = 0.125 to a point defect reaction, in which Fe3+ gains a strong influence on the development of the lattice parameters.