The semicrystalline structure and degree of crystallinity of fractionated crystallizing poly(methylene oxide)/(polystyrene/poly(2,6-dimethyl-1,4 phenylene ether) PONY(PS/PPE) blends have been investigated by DSC, SAXS and WAXD. The three techniques yielded highly correlated results. The degree of crystallinity of the POM phase determined by DSC (X-c,X-DSC) decreases with decreasing POM content in the blends and this is accompanied by a shift from bulk to homogeneous crystallization. The reduction in the measured degree of crystallinity determined by WAXD (X-c,X-WAXD) is even more pronounced and indicates, in absence of evidence for the formation of different polymorphs, that only small and imperfect crystals are formed during homogeneous crystallization in finely dispersed droplets. Analysis of the width of the WAXD reflections, which is also related to X-c,X-WAXD, yields a linear correlation between L-1, a measure of the lateral dimensions of the crystallites, and the average dispersed particle diameter. The parameter L-2, corresponding to the crystalline lamellar thickness, is non-linearly correlated with the degree of crystallinity, indicating that the decrease in X-c,X-WAXD is not solely due to the formation of thinner lamellae at higher degrees of undercooling. There is a simple relationship between the SAXS long period and the crystallization temperature, corresponding to the formation of thinner and less perfect crystalline lamellae during fractionated crystallization at higher degrees of undercooling. As the lateral dimensions of the crystallites of finely dispersed crystallizing droplets is governed by their size, X-c,X-WAXD can be directly related to the particle diameter, since the fraction of small or imperfect crystallites will not be measured by WAXD. (C) 2003 Elsevier Science Ltd. All rights reserved.