Fracture mechanisms in hot-isostatically pressed (HIP) Si3N4/SiC-platelet composites have been investigated by transmission electron (TEM) and scanning electron (SEM) microscopy followed by profilometric analyses. Two composites containing 25 vol% platelets were compared. They were fabricated from the same raw materials and by the same procedure except for the cooling rate from the sintering temperature. The study consists of experimental observations as well as measurements of fractographic parameters which dictates the level of toughening, such as the percentage of intergranular fracture, lengths and angles associated with the debonding process at the matrix/platelet interface. The presence of microcracking in the neighbourhood of the main crack, a higher fraction of intergranular fracture, as well as substantial debonding at the nitride/carbide interface up to high orientation angles were found in the composite cooled at low rates (similar to 100 degrees Ch(-1)) which, despite the unchanged microstructure, was substantially tougher than that cooled at similar to 650 degrees Ch(-1). These trends were not observed in the composite subjected to fast cooling. The stronger interfacial bonding found after fast cooling under high pressure was attributed to an apparent compressive stress remaining stored at the grain boundary, rather than to a weakening of the platelets or the matrix grains. Calculations based on the mechanics analysis of crack/interface interactions and on quantitative profilometric data, indicated a difference of about one order of magnitude in the apparent interface fracture energy of the two composites.