Biocompatible and biodegradable colloidal drug delivery systems can be obtained by means of in situ polymerization of alkylcyanoacrylate. In particular, nanocapsules of polyethylcyanoacrylate (PECA) were prepared by adding the monomer to an organic phase, consisting of Miglyol 812 and an organic solvent (ethanol, acetone or acetonitrile), and subsequently mixing the organic phase with an aqueous phase containing Pluronic F68 at different concentrations. The possible mechanism of formation and the influence of preparation conditions on the quality of nanocapsule formulations were investigated by freeze-fracture electron microscopy and laser light scattering using both the inverse Laplace transform and the standard cumulant analysis for data fitting. High-quality nanocapsule systems were obtained using an aprotic fully water-miscible organic solvent such as acetone. The presence of ethanol led to the formation of both nanospheres and nanocapsules. The concentrations of nonionic surfactant in the aqueous phase of monomer in the organic phase did not influence the kind of colloidal suspension obtained. The oil simply plays the role of monomer support. The diameter of PECA nanoparticles (nanospheres and nanocapsules) ranged from 100 to 400 nm. Three antiepileptic drugs (Ethosuximide, 5,5-diphenyl hydantoin and carbamazepine) were entrapped in PECA nanocapsules. The loading capacity of PECA nanocapsules, prepared using acetone as organic solvent, varied from 1% to 11% (drug/dried material) as a function of the solubility (affinity) of the different drugs with the oil core. This parameter also influenced the release from PECA nanocapsules, which was slower for drugs with a higher affinity for Miglyol 812. By encapsulating the three antiepileptic drugs in the PECA nanocapsules, it was possible to achieve controlled drug release. The mechanism of drug release from PECA nanocapsules was mainly diffusion from the oil core through the intact polymer barrier.