The relation between electrolysis voltage and the other variables of an electrocoagulation process was analyzed. Theoretical models describing such a relation were established. Experiments were conducted to confirm the theoretical analysis and to determine the constants in the models. Both the theoretical analysis and experiments demonstrated that water pH and flow rate had little effects on the electrolysis voltage within a large range. The electrolysis voltage depends primarily on the inter-electrode distance, conductivity, current density and the electrode surface state. The models obtained can be used to calculate the total required electrolysis voltage for an electrocoagulation process.