The kinetics of the enzymatic degradation reactions of the naturally produced polyesters poly(beta-hydroxybutyrate) (PHB) and poly(beta-hydroxybutyrate-co-beta-hydroxyvalerate (PHBV) by appropriate depolymerases were studied. Specially, the interaction between the extracellular PHB depolymerase of Pseudomonas lemoignei and both PHB powder and solution-cast films of PHBV was investigated. Kinetic analysis of titristatically and turbidimetrically determined rates of degradation has revealed that the observed degradation behaviour was inconsistent with classical Michaelis-Menten enzymatic kinetics. Depending upon the relative concentrations of enzyme and substrate, each exhibited a range of kinetic dependencies from zero order to first order. At high relative concentrations of enzyme, the reaction rate became inversely proportional to enzyme concentration. A new kinetic treatment was derived which predicted the observed variations in kinetic dependence on both enzyme and substrate, and it successfully described previously published kinetic data for this and other PHB depolymerases. The degradation of PHB by the PHB depolymerase of Aspergillus fumigatus M2A was investigated for comparison. While the specific rate constants of this enzyme differed from that of P. lemoignei, the general kinetic model was still applicable.