The results gained from the study of cobalt accumulation on polyethylene are presented in this paper. To clarify the role of colloid formation, the presence of berate species, and the surface properties of polyethylene in accumulation process, several independent techniques such as the in situ radiotracer method, centrifugation, FTIR, and Raman spectroscopy are used. From the results, it can be stated that the kinetics of cobalt accumulation is dependent on the pH and composition of the solution phase. The accumulation of cobalt proceeds in both colloid and ionic forms; however, the amount of sorbed ionic cobalt species increases with pH due to their accumulation on colloids. The lower cobalt accumulation in berate-containing solution is attributed to the lower amount of colloid formation.