Different compositions of Y- and Nd-alpha-SiAlONs were sintered at 1825 degreesC and analysed by the XRD method. The Rietveld technique was used for characterisation of the diffraction pattern and the x value of alpha-SiAlON, which has a composition of Rx+vSi12-(m + n)Alm + nOnN16 - n (R = Y, Nd). It was shown that the method yields reliable data for the content of the rare earth cations in the alpha-SiAlON phase, even in mixtures of alpha-SiAlONs with beta- or alpha-Si3N4, but cannot be used for the determination of the Al and/or O contents of the alpha-SiAlONs. On the basis of these data, the stability regions of Y- and Nd-alpha-SiAlONs were established. Furthermore, it was found that the solubility area of the alpha-SiAlONs depends on the size of the additives. It was also shown that the solubility area of the Y-alpha-SiAlON is extended to lower X values than those suggested in the literature. (C) 2002 Elsevier Science Ltd. All rights reserved.