Micron-sized and microencapsulated drugs are desirable in the pharmaceutical industry for drug targeting and controlled release systems. The current methods available for micronisation and microencapsulation, however, are limited by various factors. In this study, the feasibility of using dense CO2 with the technique mown as the Aerosol Solvent Extraction System (ASES) to micronise and microencapsulate parahydroxybenzoic acid (p-HBA) and lysozyme, with a bioerodible polymer, poly(L-lactic acid) (L-PLA), from various organic solutions, was examined. In the micronisation studies, the effects of various parameters, such as pressure, temperature, solution concentration, solvent system and spraying velocity, on the nature of the particles were determined. Effective size reduction of the particles was achieved at low to moderate temperatures in a fundamental one-step process. In general, it was found that the high-molecular-weight compounds, L-PLA and lysozyme, precipitated as microspheres and nanospheres, whereas the lighter-weight compound, p-HBA, precipitated as crystalline particles resembling platelets averaging 3 gm in length. The feasibility of micro encapsulating p-HBA and lysozyme with L-PLA using the ASES process incorporating a multiple nozzle was then assessed. Various parameters, such as the flow configuration, drug-to-polymer ratio, pressure, temperature and nozzle geometry, were varied to examine their effects on the size and morphology of the particles formed, particle drug loading and encapsulation efficiencies. The drug loading and encapsulation efficiency of particles can be improved by changes to the operating conditions. The maximum encapsulation efficiencies for p-HBA/L-PLA particles obtained in this work were 9.2%. Higher encapsulation efficiencies of 15.6% were achieved with lysozyme, possibly due to its smaller particle size upon precipitation from dimethylsulfoxide solvent. Whilst loadings are lower than those achieved in some conventional encapsulation techniques, the results suggest that improvements can be obtained by maximising the contact between the drug and polymer phases during the rapid precipitation process.