Differential scanning calorimetry and dynamic mechanical analysis of blends of a new thermoplastic polyimide (TPI) and poly(ether imide) (PEI) have confirmed the full miscibility of the system over the whole composition range. Annealing of the blends above the glass transition temperature of TPI, but below its crystallization temperature, did not produce a shift in the glass transition, while physical ageing of the annealed blends also failed to reveal any indication of phase separation. The rate of crystallization of TPI was slowed by the addition of PEI, and the temperature of the maximum crystallization rate shifted upwards. The beta-relaxation behavior of the blends followed a linear trend between the response of the component polymers, while the low-temperature gamma-relaxation was unchanged by blending. The time scale of physical ageing of the blends did not behave in a linear fashion, and the enthalpy loss on ageing also deviated from the average.