In this study, an assessment of the mechanical performance of a newly developed carbon fibre-reinforced polyimide composite system T650/NEXIMID® MHT-R is presented. This system was subjected to a series of mechanical tests at ambient temperature in order to determine the tensile, compressive, flexural and interlaminar shear properties. Moreover, an additional testing campaign was conducted, using a T650/NEXIMID® MHT-R laminate in which the sizing had been thermally removed prior to manufacturing, in order to investigate the effect of fiber treatment on the mechanical performance. The experimental results indicated that the T650/NEXIMID® MHT-R composites along with exceptionally high Tg (~370-420ºC) exhibited very good elastic properties in comparison with other polyimide and epoxy-based systems and, although slightly lower than the best results from literature, promising strength values. Finally, the thermal removal of the sizing did not affect the tensile, compression and flexural properties, however the interlaminar shear strength was significantly deteriorated.