The preparation of nanoparticles by emulsion solvent evaporation is a very popular method. The purpose of the present study was to clarify the mechanism by which nanoparticles of ethylcellulose (EC) and poly(lactic acid) (PLA) are formed during the emulsion solvent evaporation procedure. This study was mainly based on the measure of the variation of the emulsion and nanoparticle surface charge and size during the solvent evaporation process. From the data obtained and depending on the polymer used (EC or PLA), two different models are proposed to explain the nanoparticle formation. In the EC model, after shrinkage of the emulsion droplets as the direct consequence of solvent evaporation, coalescence occurred before stable and solvent-free nanoparticles were formed. On the contrary, in the PLA model, no or limited coalescence was found to occur so that the picture is that one PLA nanoparticle originated from one (or only a few) PLA emulsion droplet after its shrinkage.