The crystallization of poly(vinylidene fluoride) (PVDF)/poly(3-hydroxbutyrate) (PHB) blends was studied with differential scanning calorimetry, from which the phase diagram was derived. Strong miscibility was underlined by the large negative Flory-Huggins interaction parameter (-0.25). The crystallization of the blend components differed remarkably. Whereas PVDF always crystallized in the Surrounding of a homogeneous melt, PHB crystallized in a volume that was confined by the already existing PVDF spherulites, partly in their surrounding and partly inside. Under isothermal conditions, PVDF usually crystallized regularly in three dimensions with predominant quench-induced a thermal nucleation. The Avrami exponent for PVDF dendritic spherulitic growth was, however, distinctly smaller than that for compact growth, and this revealed the two-dimensional lamellar growth inside. This deviation from ideal Avrami behavior was caused by the development of compositional inhomogeneities as PVDF crystallization proceeded, and this decelerated the kinetics. PHB crystallized three-dimensionally with mixed thermal and a thermal nucleation outside the PVDF spherulites. Inside the PVDF spherulites, PHB crystallization proceeded in a fibrillar fashion with thermal nucleation; the growth front followed the amorphous paths inside the dendritic PVDF spherulites. The crystallization was faster than that in the melt of uncrystallized PVDF. Solid PVDF acts possibly heterogeneously nucleating. accelerating PHB crystallization. (C) 2004 Wiley Periodicals. Inc.