Previous studies on single fibre composites using micro-Raman spectroscopy revealed that the primary interfacial degradation mechanism in graphite/epoxy composites after hydrothermal exposure is mechanical in nature. Mechanical degradation of the interface is the result of a complex state of stress created at the interface due to matrix swelling upon water absorption. The major component of such stresses is a radial tensile stress which causes a tensile failure of the interfacial bonds during exposure. To date, the effect of fibre volume fraction on the state of stress created at the interface due to matrix swelling has not been considered. In this paper, finite element analysis (FEA) is used to model bulk composites with different fibre volume fractions to determine the applicability of single fibre composite behaviour to that of bulk composites. The analysis showed that the mechanical degradation mechanism can operate at all possible volume fractions and that in the case of a non-homogeneous fibre distribution, regions with both very high or very low fibre content are more susceptible to environmental degradation. Experimental results obtained from bulk composites (Vf≈63–71%) confirmed the FEA results and showed that the interfacial degradation in that case was more severe than in the case of single fibre composites.