A process for the preparation of nanoscale particulate material is described comprising: (i) combining one or more functional material to be precipitated as nanoscale particles and one or more surface active material in a compressed CO.sub.2 phase with a density of at least 0.1 g/cc, where the functional material is substantially insoluble in the compressed CO.sub.2 in the absence of the surfactant, the surfactant comprises a compressed CO.sub.2-philic portion and a functional material-philic portion, and the compressed CO.sub.2 phase, functional material and surfactant interact to form an aggregated system having a continuous compressed CO.sub.2 phase and a plurality of aggregates comprising surfactant and functional material molecules of average diameter less than 10 nanometers dispersed therein; and (ii) rapidly depressurizing the compressed CO.sub.2 phase thereby precipitating the dispersed functional and surfactant materials in the form of composite particles of average diameter from 0.5 to less than 10 nanometers.