A new kinetic model is proposed for the accumulation of poly(3‐hydroxybutyrate) (PHB) in Alcaligenes eutrophus. The model developed is closely related to the descriptive model of Harkins for the emulsion polymerization process. The PHB accumulation is discussed in terms of polymerization events such as initiation, propagation and termination, taking into account the heterogeneous nature of the system. Experimental results on the PHB accumulation in Alcaligenes eutrophus can be described in both a quantitative and qualitative manner by the model. Further, it is shown that the decrease in the rate of polymerization during the PHB accumulation can be explained by coalescence of the granules. The decrease of the molecular weight of the polymer formed during the accumulation process can be fully explained in terms of the kinetic mechanisms of progapation and chain transfer, and the effect of granule size on these.