A process for preparing microparticles comprising a biologically active material and a polymer and having a mean particle size expressed as the volume mean diameter of from 10 to 500 μm, wherein the biologically active material is substantially insoluble in the polymer, which process comprises: a. contacting a mixture of the biologically active material or a precursor thereof, the polymer or a precursor thereof and a processing aid with a supercritical fluid which is capable of swelling the polymer under temperature and pressure conditions necessary to maintain the fluid in a supercritical state; b. allowing the supercritical fluid to penetrate and liquefy the polymer, whilst maintaining the temperature and pressure conditions so that the fluid is maintained in a supercritical state, and blending or mixing the supercritical fluid and liquefied polymer; c. removing the blended mixture from a mixing chamber that is under supercritical conditions into a separate container that is not under supercritical conditions through a nozzle or like orifice, wherein the processing aid is selected from: (i) fatty acid esters consisting of polyglycol mono and di-esters of 12-hydroxystearic acid and of about 30% free polyethylene glycol, and sorbitan monooleate; (ii) 2-pyrrolidone, N-methyl-2-pyrrolidone and polymers of pyrrolidones; (iii) polypropylene glycol; (iv) medium chain mono-, di- and tri-glycerides having a formula (CH 2 OR 1)(CH 2 OR 2)(CH 2 OR 3) wherein R 1, R 2 and R 3 are independently H or -C (CH 2) n CH 3 (where n = 6 to 8), provided that not all R 1, R 2 and R 3 = H; and (v) poloxamers having a general formula HO(C 2 H 4 O) a (C 3 H 6 O) b (C 2 H 4 O) a H, wherein a is typically from 2 to 130 and b is typically from 15 to 67. a. contacting a mixture of the biologically active material or a precursor thereof, the polymer or a precursor thereof and a processing aid with a supercritical fluid which is capable of swelling the polymer under temperature and pressure conditions necessary to maintain the fluid in a supercritical state; b. allowing the supercritical fluid to penetrate and liquefy the polymer, whilst maintaining the temperature and pressure conditions so that the fluid is maintained in a supercritical state, and blending or mixing the supercritical fluid and liquefied polymer; c. removing the blended mixture from a mixing chamber that is under supercritical conditions into a separate container that is not under supercritical conditions through a nozzle or like orifice, wherein the processing aid is selected from: (i) fatty acid esters consisting of polyglycol mono and di-esters of 12-hydroxystearic acid and of about 30% free polyethylene glycol, and sorbitan monooleate; (ii) 2-pyrrolidone, N-methyl-2-pyrrolidone and polymers of pyrrolidones; (iii) polypropylene glycol; (iv) medium chain mono-, di- and tri-glycerides having a formula (CH 2 OR 1)(CH 2 OR 2)(CH 2 OR 3) wherein R 1, R 2 and R 3 are independently H or -C (CH 2) n CH 3 (where n = 6 to 8), provided that not all R 1, R 2 and R 3 = H; and (v) poloxamers having a general formula HO(C 2 H 4 O) a (C 3 H 6 O) b (C 2 H 4 O) a H, wherein a is typically from 2 to 130 and b is typically from 15 to 67. (i) fatty acid esters consisting of polyglycol mono and di-esters of 12-hydroxystearic acid and of about 30% free polyethylene glycol, and sorbitan monooleate; (ii) 2-pyrrolidone, N-methyl-2-pyrrolidone and polymers of pyrrolidones; (iii) polypropylene glycol; (iv) medium chain mono-, di- and tri-glycerides having a formula (CH 2 OR 1)(CH 2 OR 2)(CH 2 OR 3) wherein R 1, R 2 and R 3 are independently H or -C (CH 2) n CH 3 (where n = 6 to 8), provided that not all R 1, R 2 and R 3 = H; and (v) poloxamers having a general formula HO(C 2 H 4 O) a (C 3 H 6 O) b (C 2 H 4 O) a H, wherein a is typically from 2 to 130 and b is typically from 15 to 67.