Coprecipitation using a supercritical antisolvent technique seems to be a promising way for the elaboration of controlled herbicide delivery system since it allows the preparation of a herbicide (diuron) loaded in amorphous microparticles of a biodegradable polymer (L-polylactic acid, L-PLA). Coprecipitation was successfully realized in methylene chloride. The concentrations of the two components had a major influence upon morphology and shape of the final product. Spherical particles of L-PLA entrapping diuron were obtained provided that the initial concentrations of L-PLA and diuron did not exceed, respectively 3 wt.% and 0.1 wt.% in methylene chloride at 10 MPa and 308 K. A parallel study of the precipitation of diuron by the SAS process showed the formation of long needle-like crystals with a mean length of 500 mum. The precipitation time being very short during the SAS process, the growth rates observed are 10 000 times higher than the growth rates of diuron in aqueous solution. (C) 2001 Elsevier Science B.V. All rights reserved.