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Poly-[(R)-3-hydroxybutyrate] (PHB) films prepared by compression molding and solvent casting, respectively, were degraded with the intracellular depolymerase enzyme natively synthetized by the strain Bacillus megaterium. Quantitative analysis proved that practically only (R)-3-hydroxybutyric acid (3-HBA) forms in the enzyme catalyzed reaction, the amount of other metabolites or side products is negligible. The purity of the product was verified by several methods (UV-VIS spectroscopy, liquid chromatography, mass spectroscopy). Degradation was followed as a function of time to determine the rate of enzymatic degradation. Based on the Michaelis-Menten equation a completely new kinetic model has been derived which takes into consideration the heterogeneous nature of the enzymatic reaction. Degradation proceeds in two steps, the adsorption of the enzyme onto the surface of the PHB film and the subsequent degradation reaction. The rate of both steps depend on the preparation method of the samples, degradation proceed almost twice as fast in compression molded films than in solvent cast samples. The model can describe and predict the formation of the reaction product as a function of time. The approach can be used even for the commercial production of 3-HBA, the chemical synthesis of which is complicated and expensive.