Decolorization of an azo-dye, Reactive Yellow 15 (RY15), by coagulation process was investigated using Box-Behnken statistical experiment design and response surface analysis. Dyestuff concentration (RY 15), chitosan concentration and settling time were selected as independent variables in Box-Behnken design while color and total organic carbon (TOC) removals were considered as the response functions. Color removal increased with increasing chitosan concentration and settling time up to a certain level. High concentrations of chitosan adversely affected the color removal rates. TOC removal required more chitosan concentration but lower settling time as compared to color removal. The optimal chitosan concentration and settling time varied depending on the initial dyestuff level. For the highest dyestuff concentration tested, the optimal dyestuff concentration/chitosan concentration/settling time ratio resulting in maximum color removal (66%) was predicted to be 250 mg/L/100 mg/L/80 min, while this ratio was 250 mg/L/150 mg/L/30 min for maximum TOC removal (75%).