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.