Several semicrystalline poly(aryl ether ketone)s (PEK, PEEK and different isomers of PEKK) were melt-blended at T > 400 degrees C with three high temperature thermoplastic polyimides. The compatibility and crystallization properties of the blends were studied by differential scanning calorimetry (d.s.c.). Blends of a semicrystalline New-Thermoplastic PolyImide (N-TPI) showed some surprising differences in the fast-quenched amorphous samples. Its compatibility could be approximately ranked in the order PEKK > PEK much greater than PEEK, where PEEK was completely incompatible. For the case of PEK/N-TPI, extensive d.s.c. data on amorphous fast-quenched films blended at different temperatures between 400 and 455 degrees C were used to construct a phase diagram showing an upper critical temperature of 440 degrees C. Two other wholly aromatic non-crystalline polyimides, LARC-IA and LARC-ITPI, were also studied. These are difficult to process in their neat form due to high melt viscosity. In the case of LARC-IA, the addition of miscible or semi-miscible lower viscosity polymer such as PEKK lowers the melt viscosity dramatically. LARC-ITPI was completely incompatible in all cases. N-TPI and LARC-IA were also melt blended with different isomers of PEKK including PEKK(50T/50I), (60T/40I), (80T/20I) and (100T/0I), which have melting temperatures of 320, 310, 355 and 390 degrees C, respectively, and equivalent glass transition temperatures of similar to 160 degrees C; T/I refers to the ratio of 1,4 and 1,3 ketone linkages, respectively. Surprising differences were seen in the miscibility of the different PEKK isomers. PEKK(60T/40I) was the only PEKK miscible with N-TPI and LARC-IA at essentially all compositions. Tentative structure-property relationships for blend compatibility and the nature of the interactions leading to miscibility are discussed.