Agglomerate-free SiCp-reinforced Ln-sialon (Ln=Nd and Yb) composites were fabricated by hot-pressing and by pressureless sintering to evaluate the improvement in microstructure and mechanical properties that could be achieved by incorporating sub-micron SiC particles into a sialon matrix and then inducing alpha --> beta sialon transformation by heat-treatment. Both Nd2O3 and Yb2O3 were successful in producing dense samples by hot-pressing, but Yb2O3 produced a more stable or-sialon phase than Nd2O3. As a result, Yb-sialon/SiCp composites showed higher hardness (H-v), attributable to the higher percentage of alpha-sialon grains (formed in acicular morphology) in these materials. Under comparable heat-treatment conditions, the alpha-sialon phase present in Nd-sialon/SiCp composites was very unstable and almost completely transformed to beta-sialon, with the aluminium-containing melilite phase (M') forming at pockets in grain junctions as the dominant grain-boundary phase. Even though the Nd-densified samples contained a large proportion of beta-sialon, the fracture toughness(K-Ic) was not significantly improved by SiCp addition because of the decrease in aspect ratio and the coalescence of grains caused by the longer heat-treatment which resulted in degradation of mechanical properties, especially K-Ic. (C) 1997 Elsevier Science Limited. All rights reserved.