The effect of adding a nonadsorbing charged polymer, sodium polystyrene sulfonate (SPSS), on the interaction energy profile between a 15 mu m diameter polystyrene particle and a glass plate was measured using the optical technique of total internal reflection microscopy. A typical profile consisted of a repulsive barrier at large separations followed by a secondary well. Evidence of a longer-range tertiary well was also observed in salt-free solutions where the polymer molecules are rod-like. Addition of electrolyte causes a reduction in the depth of the tertiary well and produces oscillations at large separations. In addition, the stability of a solution of charged polystyrene particles in the presence of SPSS was measured using optical light scattering. Because of the electrostatic repulsion between the particles, the solution was stable against primary flocculation with no added polymer. However, because of long range attractive depletion forces, reversible secondary flocculation occurred into a local potential energy minimum at low SPSS concentrations (order 0.2 wt%). The polymer caused flocculation at a critical flocculation concentration (upsilon*) which was found to decrease with increasing particle size. No restabilization of the latex dispersions was observed.