The morphology of extruded films based on blends of polyvinyl alcohol (PVA) and poly(3-hydroxybutyrate) (PHB) was studied for various compositions. The methods of differential scanning calorimetry (DSC) and X-ray analysis were used. The phase-sensitive characteristics of the composite films, diffusion and water vapor permeability were also investigated. Processes of binding of water and swelling cause the first areas; processes of a relaxation and transition of structure of composites to an equilibrium condition, the second. In addition, the tensile modulus and relative elongation-at-break were measured. Changes in the glass transition temperature (Tg) of the blends and constant melting points of the components show their partial compatibility in intercrystalline regions. At a content of PHB in the composite films equal to 20–30% wt., the mechanical characteristics and water diffusion coefficients are dramatically changed. This fact, along with the analysis of the X-ray diffractograms, indicates a phase inversion in the above narrow concentration interval. The complex pattern of the kinetic curves of water vapor permeability is likely to be related to additional crystallization, which is induced in the composite films in the presence of water.