Abstract: Controlled release microspheres of hydroxybutyrate polymer comprise active ingredient and hydroxybutyrate/hydrovalerate copolymer. The microspheres comprise a skin which is distinct from the general bulk of the microspheres, and this skin may have a porosity covering from 0-50% of the total surface area of the microsphere, this porosity being largely controlled by varying the hydroxyvalerate content of the polymer. The porosity of the interior of the microspheres can also be regulated. Control of the two porosities permits the attainment of a wide range of release rates for a wide range of active ingredients. The microspheres can be used in a wide variety of pharmaceutical, veterinary and agricultural applications.