Kinetics of lamellar microstructure development of poly(aryl ether ether ketone) (PEEK), poly(aryl ether ketone ketone) containing 70% isophthalate moieties (PEKK(30/70)) and their 50/50 blend during isothermal crystallization and subsequent melting have been studied by DSC, transmitted light intensity, and time-resolved small-angle X-ray scattering (SAXS). The crystallization peak time, spherulite growth rate, and dimensions of lamellar stacks (long period L, crystal lamellar thickness l(c) and amorphous layer thickness I-a) were determined. It was found that there is a remarkable difference in the crystallization kinetics of pure PEEK and PEKK(30/70). In the blend, the addition of PEKK(30/70) significantly reduces the crystallization rate of PEEK, resulting in a peak time intermediate between the two neat materials. A two-step behavior was observed during isothermal crystallization and subsequent melting processes in the blend by both transmitted light intensity and time-resolved SAXS techniques. Results show no sign of cocrystallization or epitaxial growth between the crystals of PEEK and PEKK(30/70) rather than the coexistence of two separate lamellar structures (PEEK and PEKK(30/70)) in the same spherulites. It is thus concluded that during crystallization, PEEK spherulites formed first followed by the infilling of PEKK(30/70) crystals within these spherulites.