Composites of amoxicillin and ethyl cellulose in the micrometer range were precipitated by supercritical antisolvent (SAS) processes using carbon dioxide as antisolvent and a mixture of dichloromethane and dimethylsulfoxide as solvents. Morphologies and mean diameter ranges were analyzed by scanning electron microscopy. X-ray photoelectron spectroscopy (XPS) and high-performance liquid chromatography were carried out in order to ensure successful coprecipitation and to determine the amoxicillin contents in the final products. The SAS processes used differ mainly in the way in which the solutions are introduced: through a normal or a coaxial nozzle. The XPS results provided proof that amoxicillin was not distributed in the same way in all the samples. The release behavior of the composites obtained was evaluated in two biological fluids, i.e., simulated gastric and simulated intestinal fluids. The different systems led to the release of the drug in different ways; but in all cases slower solubilization was obtained than for the pure drug.