Submicron particles were produced by rapid expansion of supercritical solution into air (RESS) or an aqueous surfactant solution (RESSAS) to minimize particle growth and to prevent particle agglomeration. Thereby the effect of process conditions on the size of the particles precipitated was investigated. The obtained product was evaluated by measuring particle size by 3-wavelength extinction measurements, dynamic light scattering, specific surface areas by nitrogen gas adsorption, melting behaviour by differential scanning calorimetry, particle morphology by X-ray diffraction, scanning electron micrographs (SEM), and drug loading by high performance liquid chromatography. Prior to the particle formation experiments, the melting temperature of Salicylic acid under CO2 pressure and the solubility of Salicylic acid in CO2 were measured. The size of Salicylic acid particles produced via RESS decreased from 230 to 130 nm as the pre-expansion temperature decreased from 388 to 328 K and the specific surface area of the micronized particles was found to be up to 60 times higher than that of the unprocessed material. RESSAS experiments demonstrate that in 1 wt.% Tween 80 solutions Salicylic acid concentrations of 4.6 g/dm(3) could be stabilized with particle diameters in the range of 180 nm. Additional experiments show that Ibuprofen nanoparticles with an average size of 80 nm and a drug concentration of 2.4 g/dm(3) could be stabilized in 1 wt.% Tween (R) 80 solutions. The use of a SDS solution instead of Tween (R) 80 results in a stable aqueous suspension of phytosterol nanoparticles, where the average particle size is 50 nm at a drug concentration of 5.6g/dm(3). (C) 2008 Elsevier B.V All rights reserved.