A fundamental study about the characterization of a poly(N-isopropylacrylamide), poly(NIPAM), microgel, cross-linked with N,N ' -methylene bisacrylamide (BA), has been carried out. The anionic charge of this system was increased by copolymerization with 2-acrylamido-2methylpropanesulphonic (AMPS) acid. The electrokinetic behavior of these aMPS/NIPAM microgel particles has been investigated. First, the surface charge density was determined by conductimetric titration, and the particle size was obtained by transmission electron microscopy under several conditions. Electrophoretic mobility measurements were carried out as a function of electrolyte concentration and temperature. The hydrodynamic diameter of the particles was also measured by photon correlation spectroscopy as a function of electrolyte concentration and temperature. These size data have been taken into account in order to explain the mobility behavior by using Ohshima's theory for soft particles, with very good agreement between experiments and theoretical predictions.