The invention provides a method for forming particles of a target substance, comprising (a) co-introducing into a particle formation vessel, under controlled temperature and pressure, a supercritical or near-critical anti-solvent fluid; a first target substance in a first vehicle; and a second target substance in a second vehicle; and (b) using the anti-solvent to disperse the target substances in their respective vehicles and to extract the vehicles, substantially simultaneously and substantially immediately on introduction of the fluids into the particle formation vessel. The second vehicle is immiscible with the first, and contact between the first and second vehicles occurs a sufficiently short period of time before their dispersion by the anti-solvent, and with sufficient physical mixing, as to allow only insignificant, if any, phase separation to occur between the two vehicles between their contact with one another and their dispersion.