Design of bioerodible composites combining synthetic and natural polymers is an important stage for development in constructional and packaging materials which are friendly environmental ones during exploitation and thrown on a garbage dump. For this object the composites on the base of poly(3-hydroxybutyrate) (PHB) and polyisobutylene (PIB) were prepared in plunger extruder. Morphology of the blends was studied by AFM (NTegra Prima) with cantilever NSG-01 with frequency 0,5-1,0 Hz and DSC (DSC-60 9Shimadzy, Japan). Rheology characteristics were obtained by plastometer Stress Tech (Rheologica Instruments, Sweden). Degradation was performed in soil at 22-25°С with following weighing to determine weight loss. Two-phase matrices of the composites show displacement of glass transition temperatures of individual phases. Comparison of experimental and theoretical values of viscosity confirmed the formation of a continuous matrix with less viscous PHB. This greatly simplifies the processing of melts of compositions PHB-PIB. Microbial degradation of PHB phase occurs almost entirely within 100-125 days. The remaining friable matrix PIB is much more susceptible to degradation by other destructive factors (oxygen of the air, temperature, mechanical stress). Process of biodegradation of PHB phase influences on the structure of PIB. This is reflected in a sharp decrease of average molecular weight of PIB.