Aqueous- core capsules with uniform polymeric shells and diameters ranging from 0.2 to 5 Am were prepared by polymerizing the interfaces of inverse emulsion microspheres. Free- radical polymerization was constrained to the interface of water- in- oil microspheres by the alternating copolymerization of hydrophobic maleate esters and hydrophilic polyhydroxy vinyl ethers, in a manner analogous to classical interfacial polycondensations. In these polymerizations, the kinetics, shell thickness, and release characteristics of the resulting aqueous- core capsules are set by the diffusion- limited alternating reaction of the oil- soluble maleate esters and water- soluble vinyl ethers.