The phase relationships along binary join of SiO₂-AlPO₄ were investigated up to 400°C using several starting materials made by a partial solution route. Precursors used were boehmite (AlOOH), H₃PO₄, non crystalline silica (Ludox, Cab-O-Sil), and quartz. Studies up to 400°C showed that SiO₂, AlPO₄, and its hydrate were the only crystalline and non-crystalline phases present along the binary join, and no substantial crystalline solution of any ternary phase was observed. Three polymorphic forms of AlPO₄, namely, berlinite, tridymite form, and cristobalite form, coexisted at as low as 200°C. The nature of the silica precursors greatly influenced the development of the polymorphic phases of AlPO₄. The low quartz precursor suppressed the formation of the cristobalite form of AlPO₄ and favored berlinite production. On the other hand, non crystalline silica with a cristobalite-like broad XRD peak suppressed the formation of berlinite and enhanced that of the cristobalite form of AlPO₄. The silica precursors acted as structural seeds for the growth of AlPO₄. These precursor effects indicate that heteroepitaxy is very significant during the nucleation and growth of AlPO₄ phases on the surface of SiO₂ particles even in these low temperature reactions. The influence of other precursors and route during the syntheses of AlPO₄ and other phases in the SiO₂-AlPO₄ are discussed.