The preparation of monodisperse emulsions of ''silicone oil'' (poly(dimethylsiloxane) (PDMS)) in water has been carried out via the base-catalyzed hydrolysis and polymerization of dimethyldiethoxysilane (DMDES) in water or water/ethanol mixtures. This method is analogous to the nucleation and growth of colloidal silica particles. The physical properties of the emulsions have been studied using dynamic light scattering and electrophoresis. Characterization of the PDMS phase has been undertaken principally by Si-29 and H-1 NMR, mass spectrometry, and GPC. The results show that in the absence of ethanol the cyclic tetramer octamethylcyclotetrasiloxane (D4) is the most prominent polymeric species. The ratio of linear to cyclic polymer in the PDMS phase may be altered by changing the amount of alcohol and/or monomer present in the initial reaction mixture. We also report the preparation of aqueous/PDMS emulsions by addition of water to an alcoholic solution of low molecular weight PDMS. (C) 1994 Academic Press, Inc.