The bulk tensile behaviour of poly(ether ether ketone) and poly(ether imide) homopolymers and their blends has been investigated, and the temperature and strain rate dependence of the yield stress discussed in terms of simple Eyring rate theory. In fracture mechanics tests, the K-IC of PEEK was found to decrease significantly with increasing test speed, whereas the K-IC of PEI showed little rate sensitivity. Therefore, although a gradual increase in K-IC with increasing PEEK content was observed in the blends at low loading rates, this effect was anticipated to be less pronounced at higher loading rates.