In this study, the effects of supporting electrolyte type and concentration were investigated on the batch removal of water with high concentration of natural organic matter (NOM) by the electrocoagulation method using plate electrodes. It was observed that the addition of supporting electrolyte has a twofold effect which can be summarized as follows: firstly increasing the ionic strength of the water causing compression of the double-layer and secondly raising the electrical conductivity of the water causing more current passing through the circuit under the same applied potential. It was observed that the concentration of 5 mM supporting electrolyte was optimum up to 100 mg/L of NOM, while concentration of 10 mM is more favorable for higher concentrations than 100 mg/L of NOM. In order to determine the most favorable supporting electrolyte type, NaCl, Na2SO4 and NaNO3 were used as supporting electrolyte in the test runs. Under the conditions of initial pH of the solution equal to 5.0 and initial concentration of 100 mg/L NOM, charge loadings were within the range of 2.0934-3.7437 F/m(3) for Na2SO4 and NaCl, respectively. Additionally at the same initial conditions for a treatment period of 9 min specific energy consumptions and removal efficiencies were ranged between the 4.747 and 9.695 kWh/m(3), and between 96.84% and 99.64% for Na2SO4 and NaCl, respectively. Availability of the chloride (Cl-) ions in the water can cause formation of disinfection by-products (DBPs) which are suspected carcinogenic compounds. It can be said that the most favorable supporting electrolyte type is Na2SO4 for this treatment technique. (C) 2007 Elsevier B.V. All rights reserved.