Mode I interlaminar fracture of a novel amorphous thermoplastic polyimide reinforced with unidirectional carbon fibre has been studied experimentally using double cantilever beam specimens and scanning electron microscopy. Three kinds of composite were manufactured from different monomeric reactant solutions which were prepared by using different alcohol solvents. The values of fracture toughness of these three composites were measured to construct the crack growth resistance curves (R curves). The contributions from various failure processes to the total fracture toughness were separated, and approximate calculations of these contributions were conducted based on several simplifying assumptions and some data obtained from the fracture surfaces. Though fibre peeling and fibre breakage are observed, interlaminar fracture in the composites studied is primarily controlled by fracture and deformation in the matrix. It is found that the measured fracture toughnesses of the composites differ from each other not only in the propagation values but also in the initial values. A possible reason for this may be variations of matrix ductility in the three composites.