The influence of particle size and particle type on the fracture toughness of aluminium matrix composites is investigated using model composites of 99.99% purity aluminium reinforced with particles of alumina and boron carbide. These composites are produced by gas pressure infiltration, and feature a volume fraction of reinforcement near 50 vol pet. The average particle size is varied between 5 and 35 mum. Fracture properties are determined using the single specimen J-integral test method. Observations of crack propagation mechanisms are conducted by optical microscopy using the arrested crack technique. Equivalent initiation fracture toughness values (K-JC) up to 24 MPa .m(1/2) for the toughest composites are measured, a comparatively high value for a material consisting of 50 vol pet ceramic. In both composites systems, it is found that the average particle size exerts a significant influence on fracture initiation and crack propagation resistance. Microscopical investigations of the mode of fracture show that cleavage of angular particles is present, and becomes increasingly dominant in alumina reinforced composites as the average particle size increases. For composites reinforced by rounded alumina or angular boron carbide particles, matrix voiding is dominant. Particle cracking results in a lower fracture initiation roughness, while increasing particle diameter results in increased crack initiation toughness and tearing modulus.