PEI/PEEK blends have been prepared and analysed by DSC. The blends are compatible over the full composition range in that amorphous samples are transparent and exhibit a single Tg varying with composition between the limits of the two components. The crystallisation kinetics of PEEK from the blend melt has been measured by DSC but the technique is limited to blend compositions above 20% PEEK. The use of a polarised light microscope to measure crystallization kinetics and melting behaviour of blends with compositions as low as 2 and 5% PEEK is discussed. In general, since the extent rather than the rate of crystallization is measured directly, the polarised light microscopy extends the measurement to slower rates and so to a wider temperature range.