The temperature of the dielectric β-transition of poly(vinylidene fluoride) (PVDF), which is generally assigned to the glass temperature of the liquidlike amorphous phase of PVDF, is found to remain invariant in its compatible blends with poly(methyl methacrylate) (PMMA) in which PVDF exhibits crystallinity. This invariance of the β-transition temperature over a large composition range from PVDF homopolymer to 60/40 PVDF/PMMA and its disappearance in the completely amorphous blends are explicable by recognizing the crystal-amorphous interphase associated with lamellar crystallites of PVDF. These experimental results therefore provide a compelling argument that the β-transition is not related to the liquidlike amorphous phase, but rather arises from the crystal-amorphous interphase in lamellar PVDF crystallites, where PMMA is completely excluded despite favorable segmental (mixing) interactions. The latter point derives from recent theoretical considerations which show that the interphase in lamellar semicrystalline polymers is necessitated by the difficulty in abruptly dissipating the high chain flux at the crystal surface to the level commensurate with the isotropy of the amorphous phase; the presence of PMMA in the interphase will therefore be deleterious in resolving the chain flux problem. It follows then that the strong β-transition of PVDF is indicative of a significant fraction of the interphase region in semicrystalline PVDF, which can be attributed to the presence of considerable amounts of head-to-head and tail-to-tail defects in PVDF chains according to the theory. Relevance of this interphase to dielectric constants and electric-field-induced orientation characteristics of PVDF are also discussed. © 1985, American Chemical Society. All rights reserved.