The relation between microstructural characteristics and fracture behaviour of Si3N4/SiC-particle composites were evaluated for a series of materials containing a 25 vol% dispersion, with mean size in the range 7-106 mu m All the composites were fabricated by hot isostatic pressing without external addition of sintering aids via glass encapsulation. Quantitative image-analysis techniques were employed to assess the microstructural parameters, dealing with morphology and distribution of the SiC particles. A fracture mechanics analysis based on the determination of fracture strength, toughness, work of fracture and rising R-curve behaviour provided the basis for discussion of the effectiveness of the SiC dispersions. The results of mechanical tests are compared with those obtained on the monolithic material fabricated by the same process. The microfracture mechanisms in composites are discussed by relating microstructural data, obtained by image analysis, to toughness data.