The thermal behaviour of the (NH4)3[M(III)Mo6O24H6] . 7H2O, Anderson phases, where M is Al, Co, Fe has been analysed in N2, N2/O2 and N2/H-2 atmospheres using TG-DTA, TPR, and XRD techniques. Additional measurements were made by IR, EPR and Mossbauer spectroscopies, and also by scanning electron microscopy. Similar studies were carried out on ammonium heptamolybdate for comparative purposes. Molybdenum oxide and metallic molybdate are the solid products of the thermal treatment in N2 and N2/O2 atmospheres. While M2(MoO4)3 is observed when M is Al and Fe, the beta-CoMoO4 binary oxide is stabilized under both experimental conditions. The intermediate [(NH4)2O]0.8MoO3(H2O)0.3 phase is only observed when working in static air atmosphere at a very low heating rate. The temperature-programmed reduction (TPR) study permits the analysis of the different reduction steps of the Mo(VI) and M(III) is Fe, Co components. Mo4O11, MoO2, Mo and MMoO4 (M is Co, Fe) are also observed. The formation of an Al-Mo reduced oxidic phase (related to MoO2) can be inferred for the Al compound.