The aim of the presented work was to study formation of magnesium aluminate spinel precursor powder during the co-precipitation of magnesium and aluminium nitrate with ammonium carbonate and its thermal transformation to spinel powder. After precipitation, the only crystalline phase is NH₄Al(OH)₂CO₃·H₂O (ammonium dawsonite). The second one, Mg₆Al₂(CO₃)(OH)₁₆·4H₂O (hydrotalcite) appears during ageing. As found by DTA-MS and HT-XRD measurements, co-existence of both phases in very close contact results in their easier decomposition. On the base of the results obtained the scheme of phase transformations of co-precipitated precursor leading to formation of MgAl₂O₄ is proposed.