Submicron organic particles are produced by precipitation in an emulsion. The poorly water soluble organic substance is dissolved in a non-polar solvent. This solution is dispersed in an aqueous phase in the presence of emulsifier. When the non-polar solvent is removed by evaporation, the organic substance precipitates and one particle is formed in each emulsion droplet. Experimental work are here reported on the rate and the influence of the evaporation rate on the formation of cholesteryl acetate particles. An introductory theoretical analysis of the conditions experienced by each droplet during the evaporation is given.