High temperature x-ray diffraction was used to investigate the evolution of phases at 1050 C in three AlN-based ceramic powder mixtures containing Al2O3 and Y2O3 additives. The overall composition was 87.5 mol% AlN, 10% Al2O3 and Y2O3 2.5%; the alumina precursors in the three mixtures were Al2O3, AlCl3 and Al(NO3)3 respectively. The effects of the tape of precursor and the heat treatment atmosphere (vacuum or nitrogen) on the phases formed and the sequence of their formation over a period of two hours, are presented and discussed.