The electrocoagulation of kaolinite and bentonite suspensions was studied in a pilot electrocoagulation system at the Western Research Centre of CANMET to assess the cost and efficiency of the process. Factors affecting cost such as, the formation of passivation layers on electrode plates and the recirculation and concentration of sodium chloride were examined. Colorimetry was used to analyze aluminum content in the suspension. The results were used to calculate - the cost due to the consumption of electrode material, i.e., aluminum, during the process. Total cost was assumed to comprise the energy cost and the cost of aluminum electrode material. comparison was based on the settling properties of the treated product: turbidity, settling rate, and cake height. In most cases, aluminum efficiency averaged around 200% and aluminum material cost accounted for 80% of the total cost. Experimental results also showed that although higher concentrations of sodium chloride could only slightly increase aluminum efficiency and the electrode efficiency, the higher concentrations resulted in much greater total cost. This was caused by the greater current generated due to the increased conductivity of the suspension, which in turn, dissolved a larger amount of aluminum. The recirculation loop installed on the cell system increased the flow rate by 3 to 10 times, enhancing the mass transport between the electrodes and resulting in both lower cost and better settling properties. During the two-month experiment the electrodes were coated by clayish layers which gradually became thicker, while the performance of the cell system deteriorated. The term "electrode efficiency" was defined to evaluate the change in efficiency of the - electrodes due to the formation of passivation layers. The electrode efficiency was found to be as high as 94% for virgin electrode plates and to decline to as low as 10% for the saine system operated two months later. During this time interval the current waveform became greatly distorted and less and less current was driven by the same voltage applied.