Microparticles of ethyl cellulose (EC) and amoxicillin (AMC) have been precipitated by a supercritical antisolvent process (SAS) using CO2 as the antisolvent and a mixture of dichloromethane (DCM) and dimethyl sulfoxide (DMSO) as solvents. Combinations of three temperatures (308, 323 and 333K) and four pressures (100, 150, 200 and 250bar) were assessed in the vessel and the rest of the variables were held constant (i.e. CO2 flow rate, sample flow rate, washing time, nozzle diameter and the amoxicillin:ethyl cellulose ratio). Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and elemental analysis (EA) were used to determine the particle size and shape and to confirm the presence of both compounds in the resulting precipitates. In most cases, mixed amoxicillin and ethyl cellulose particles were produced with sizes in the micrometer range. Pressure and temperature effects on the co-precipitation were investigated. The release behaviour of the microparticles precipitated by the SAS process was evaluated in two biological fluids – simulated gastric and simulated intestinal fluids. Co-precipitated materials allowed a slower drug release rate than pure drug.