Unidirectional T300/914 carbon epoxy composite has been tested under transverse tensile loading at various temperatures and test rates. The test programme revealed a different viscoelastic response for the bulk resin modulus from that of the composite modulus and associated fractographic examination showed the structures of the resin and composite to be complex. The resin consisted of an epoxy-based particulate phase embedded within a thermoplastic-based connecting phase. The addition of carbon fibres resulted in the formation of an irregular epoxy-based interphase region. A parameter, Lambda, has been defined th at easily characterizes the viscoelastic behaviour of the composite. Structural models have been successfully developed for both the resin and the composite and a cure mechanism suggested. Any plastic deformation occurring on failure was within the connecting phase and the weakest point of the composite was identified as being the particle/interphase boundary. Property modelling has successfully accounted for the interphase influence on the transverse tensile modulus and the composite transverse tensile strength was found to be higher than the bulk resin tensile strength.