The diffusiophoretic and electrophoretic motions of homogeneous suspensions of identical, charged, spherical particles in the solution of a symmetrically charged electrolyte are analyzed under conditions of small Reynolds numbers. The thickness of the electric double layers surrounding the particles is assumed to be small relative to the radius of each particle and to the gap width between two neighboring particles, but the effect of polarization of the mobile ions in the diffuse layer is taken into account. The effects of interaction among individual particles are taken into explicit account by employing a unit cell model which is known to provide good predictions for the sedimentation of monodisperse suspensions of spherical particles. The appropriate equations of conservation of electrochemical potential energies of ionic species and fluid momentum are solved for each cell, in which a spherical particle is envisaged to be surrounded by a concentric shell of suspending fluid, and the diffusiophoretic and electrophoretic velocities of the particle are calculated for various cases. Analytical expressions of these mean particle velocities are obtained in closed form as functions of the volume fraction of the particles. Comparisons between the approximate ensemble-averaged diffusiophoretic and electrophoretic velocities of a test particle in a dilute suspension and our cell-model results are made.