A method for preparing polymeric micro- or nanoparticles comprising a biologically active agent, the method comprising the steps of: (a) providing a jet impingement reactor comprising a reaction chamber defined by an interior surface of a chamber wall, wherein the reaction chamber has a substantially spheroidal overall shape, said chamber comprising - a first and a second fluid inlet, wherein the first and the second fluid inlet are arranged at opposite positions on a first central axis of the reaction chamber such as to point at one another, and wherein each of the first and the second fluid inlet comprises a nozzle; and - a fluid outlet arranged at a third position, said third position being located on a second central axis of said chamber, the second central axis being perpendicular to the first central axis; (b) providing a first stream of a first liquid composition, wherein the first liquid composition comprises an organic solution of - a polymer selected from PLA, PGA, PLGA and combinations thereof; and - the biologically active agent; (c) providing a second stream of a second liquid composition, wherein the second liquid composition is an aqueous solution; (d) simultaneously injecting the first stream at a first flow rate through the first fluid inlet and the second stream at a second flow rate through the second fluid inlet into the reaction chamber of the jet impingement reactor such as to allow the first liquid composition and the second liquid composition to mix and thereby form a third liquid composition; (e) allowing the third liquid composition to flow out of the reaction chamber through the fluid outlet; and (f) collecting the third liquid composition. (a) providing a jet impingement reactor comprising a reaction chamber defined by an interior surface of a chamber wall, wherein the reaction chamber has a substantially spheroidal overall shape, said chamber comprising - a first and a second fluid inlet, wherein the first and the second fluid inlet are arranged at opposite positions on a first central axis of the reaction chamber such as to point at one another, and wherein each of the first and the second fluid inlet comprises a nozzle; and - a fluid outlet arranged at a third position, said third position being located on a second central axis of said chamber, the second central axis being perpendicular to the first central axis; - a first and a second fluid inlet, wherein the first and the second fluid inlet are arranged at opposite positions on a first central axis of the reaction chamber such as to point at one another, and wherein each of the first and the second fluid inlet comprises a nozzle; and - a fluid outlet arranged at a third position, said third position being located on a second central axis of said chamber, the second central axis being perpendicular to the first central axis; (b) providing a first stream of a first liquid composition, wherein the first liquid composition comprises an organic solution of - a polymer selected from PLA, PGA, PLGA and combinations thereof; and - the biologically active agent; - a polymer selected from PLA, PGA, PLGA and combinations thereof; and - the biologically active agent; (c) providing a second stream of a second liquid composition, wherein the second liquid composition is an aqueous solution; (d) simultaneously injecting the first stream at a first flow rate through the first fluid inlet and the second stream at a second flow rate through the second fluid inlet into the reaction chamber of the jet impingement reactor such as to allow the first liquid composition and the second liquid composition to mix and thereby form a third liquid composition; (e) allowing the third liquid composition to flow out of the reaction chamber through the fluid outlet; and (f) collecting the third liquid composition.