Nitrogen rich neodymium-ytterbium containing multi-cation alpha-sialon starting compositions have been prepared by gas pressure sintering using "conventional" alpha-Si3N4 or plasma produced beta-Si3N4 powder. The effect of alpha-SiAlON nucleation on growth mechanism was investigated by applying different sintering cycles. X-ray diffraction studies of these compositions after sintering revealed that a-sialon was observed as a matrix phase. Microstructural characterisation of the sintered materials by using alpha-Si3N4 powder as starting composition resulted in a typical equiaxed grain morphology, as expected if the alpha-SiAlON nucleation step is missed out prior to growth. However, elongated alpha-SiAlON grains were observed by applying nucleation. In the case Of beta-Si3N4 containing starting composition, in all cases alpha-SiAlON grain morphology was observed. The effects of starting Si3N4 powders and different sintering conditions were discussed in detail.