In this study, Taguchi's method was implemented to evaluate and to optimize the experimental conditions for the preparation of polyurea and polyurethane nanoparticles using emulsion-diffusion process. The effects of the hydrophilic monomer choice (1,4-diaminobutane (BDA) or 1,4-butanediol (BDO)), the concentration of surfactant (Tween 20), the concentration of dispersant (Sodium Dodecyl Sulfate, SDS), the organic solvent volume ratio (acetone/ethyl acetate), the time of addition of SDS and the time of dilution were studied on the encapsulation yield and efficiency, and particle size distribution. Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed to characterize the chemical structure and the thermal stability of these nanoparticles. The statistical analysis revealed that the most important factors contributing to the encapsulation efficiency are percent of surfactant, acetone content, selection of hydrophilic monomer, and dilution time. Moreover, the monomer choice influences the size distribution mode as well as the mean diameter. In most of cases unimodal distribution with a mean diameter between 125 and 1083 nm were obtained when BDA was used as hydrophilic monomer. (C) 2013 Elsevier B.V. All rights reserved.