Composite microparticles of ampicillin (AMP) and ethyl cellulose (EC) have been obtained by a supercritical antisolvent (SAS) process in which carbon dioxide was used as antisolvent. Morphologies and mean diameter ranges were analysed by scanning electron microscopy (SEM) and the ampicillin loading was determined by high-performance liquid chromatography (HPLC). X-ray photoelectron spectroscopy (XPS) was used to confirm the presence and location of both compounds in the precipitates. In most cases, mixed ampicillin and ethyl cellulose particles were produced with sizes in the micrometer range. Pressure and temperature effects on the co-precipitation were also investigated. Ampicillin release was dependent on its location in the composites and the release behaviour of the microparticles precipitated by the SAS process was evaluated in two biological fluids simulated gastric and simulated intestinal fluids. (C) 2012 Elsevier B.V. All rights reserved.