The electrophoretic behavior of spherical colloidal particles is modeled theoretically. The classic analysis is extended to the case of a concentrated dispersion in which the double layer surrounding a particle can assume an arbitrary thickness, and the effect of double-layer polarization is taken into account. Also, the surface of a particle is charge regulated, which leads to a general mixed-type boundary condition, and simulates entities bearing dissociable functional groups such as biological cells and particles with a membrane layer. We show that the absolute surface potential decreases with the increase in kappa a, kappa and a are, respectively, the reciprocal Debye length and the radius of a particle. The variation of the absolute electrophoretic mobility as a function of kappa a is found to have a local maximum. The higher the surface potential the more significant the effect of double-layer polarization, and it becomes insignificant if kappa a approaches infinity. (C) 2000 American Institute of Physics. [S0021-9606(00)70114-0].