The depletion interaction may occur when nonadsorbing, or free, polymer is present in a colloidal dispersion. Under appropriate conditions, the interaction can lead to flocculation. The present work examines aggregation stability of aqueous, electrosterically-stabilized polystyrene latices in the presence of free polymer as a function of free polymer concentration, free polymer molar mass, salt concentration, and molar mass of the steric adlayer. The stability ratio, W, is determined from early-stage aggregation kinetics as measured by photon correlation spectroscopy. The effect of salt on polymer solvency is determined viscometrically. The depletion interaction potential energy is calculated using a modified version of the pragmatic model of Vincent et al.1,2 Theoretical stability ratios are calculated from the total potential energy function and compared with experimental stability ratios. The critical volume fraction for depletion flocculation is found to decrease with increasing molar mass of free polymer. Increases in the salt concentration decrease the stability due to the destruction of polymer solvency. In some cases, the stability is observed to increase upon the addition of free polymer; however, this phenomenon is found to be fundamentally different than depletion restabilization. The model successfully describes the salient features of the stability behavior.