The crystallisation of PEEK from miscible blends of PEEK and PEI has been measured and the results analysed in terms of the Avrami equation and the Hoffman and Lauritzen crystallisation theory. The melting of the crystals was analysed in terms of the Hoffman and Weeks theory and it was found that the beta values were in excess of unity. The product of the surface free energies was found and discussed in terms of the variation in blend composition.