A novel method of encapsulating an active substance in a biodegradable polymer, which comprises: a) dissolving said biodegradable polymer in an organic solvent therefor; b1) dispersing said active substance in the organic solution obtained in step a) to provide a dispersion with the active substance as the inner phase thereof; or alternatively b2) emulsifying said active substance, dissolved in water or other aqueous solvent therefor, in the organic solution obtained in step a) to provide an emulsion with the active substance as the inner aqueous phase thereof; and c) subjecting the dispersion obtained in step b1), or alternatively the emulsion obtained in step b2), to an encapsulation operation with an aqueous polyethylene glycol solution as a continuous phase to provide micro- or nanoparticles having the active substance encapsulated therein. Sustained release particles obtainable thereby.