Simultaneous wide and small X-ray scattering (WAXS and SAXS) real time experiments using synchrotron radiation were performed to investigate poly-aryl(ether ketone ether ketone ketone) (PEKEKK) samples with different molecular weights obtained by precipitation polycondensation. Isothermal melt crystallization experiments, using two crystallization temperatures, T-c (differing by 10 degrees C) for each polymer grade were carried out. The phase transition temperature from polymorphic form II to form I was investigated in the samples as obtained. The variation of the index of crystallinity during heating was shown to depend on molecular weight. The index of crystallinity and X-ray long period values of the isothermal melt-crystallized samples observed at T-c and then at room temperature depend on the T-c values. For the lowest viscosity sample the microstructure is also molecular weight dependent. Avrami analysis of the 110(I) reflection leads to the conclusion that a similar crystallization mechanism takes place for the samples investigated.