a novel method of encapsulating an active substance in a biodegradable polymer , which comprises : a ) dissolving said biodegradable polymer in an organic solvent therefor ; b 1 ) 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 b 2 ) 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 b 1 ), or alternatively the emulsion obtained in step b 2 ), 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 .