For the first time, poly(epsilon-caprolactone) and poly(ecaprolactone-co-epsilon-caprolactam) nanoparticles were successfully obtained by anionic polymerization of epsilon-caprolactone and anionic copolymerization of epsilon-caprolactone with epsilon-caprolactam, respectively, in heterophase by the miniemulsion technique. After polymerization the resulting dispersions are stable for hours in case of the pure polyester and days for the copolymer. The syntheses were carried out with different continuous phases, amounts of surfactant, initiator, and monomers. The influence of the reaction parameters on the molecular weight of the polymers and on colloidal characteristics like size and morphology of the nanoparticles were studied by dynamic light scattering, gel permeation chromatography, differential scanning calorimetry, nuclear magnetic resonance, and Fourier transform infrared spectroscopy. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4929-4937, 2010