The influence of the amorphous polymers poly(ether sulfone) (PES) and poly(ether imide) (PEI) on the crystallization of the semi-crystalline poly(ether ketone ether ketone ketone) (PEKEKK) was investigated by optical microscopy, d.s.c. and wide-angle X-ray scattering. The miscibility was estimated by measuring the glass transition temperatures. The different miscibilities of the investigated polymer systems were correlated to their crystallization behaviour. The half-time of crystallization of PEKEKK is increased in the miscible system PEKEKK/PEI whereas in the immiscible system PEKEKK/PES the half-time of crystallization runs through a maximum. Compared to pure PEKEKK the normalized crystallinity is higher in PEKEKK/PEI blends and slightly lower in PEKEKK/PES blend. The unit cell parameters of the crystalline PEKEKK phase are not changed in the blends.