Floor-wash from dye finishing plant is a major source of color and wastewater volume for dyes industries. Batch electrocoagulation (EC) of simulated floor-wash containing Reactive Black 5 (RB5) was studied as a possible pretreatment option. More than 90% of initial 25 mg/L of RB5 was removed at current densities of 4.5, 6, and 7.5 mA/cm(2) in the presence of Na(2)SO(4) and NaCl as supporting electrolytes: in less than one hour. Identical K(obs) (pseudo first-order reaction rate constant) values were obtained at initial pH of 3.74 for both electrolytes. However, at initial pH of 6.6, k(obs) values decreased in the presence of Na(2)SO(4) and remained same for NaCl as compared to that at pH 3.74. Highest extent of decolorization and k(obs) values were obtained at initial pH 9.0 for both electrolytes. Under identical conditions, specific energy consumption (SEC) was almost half in the presence of NaCI (similar to 29 kWh/kg RB5) than that of Na(2)SO(4). Vinyl sulfone (VS) was detected as one of the products of EC indicating reduction of azo bonds as a preliminary step of decolorization. Mechanism of decolorization with respect to various experimental conditions was delineated. Generation and accumulation of VS was dependent on initial pH and type of electrolyte. Results of this study revealed that EC in the presence of sodium chloride can be efficiently used as a primary treatment for decolorization of floor-wash containing RB5. (C) 2011 Elsevier B.V. All rights reserved.