The surface of coir pith, an agricultural solid waste, was modified using a catior c surfactant hexadecyltrimethylammonium bromide. Adsorption of anionic dyes on surfactant-modified coir pith was investigated in a series of batch adsorption experiments. Two anionic dyes, acid brilliant blue (acid dye) and procion orange (reactive dye), were used in the adsorption studies. The effect of process variables such as contact time, concentration of the dyes, adsorbent dose, temperature, and pH were studied in order to understand the kinetic and thermodynamic parameters of the process. The kinetics of adsorption obeyed the second-order rate equation. The equilibrium adsorption data were fitted into the Langmuir and Freundlich isotherms. It was found that modified coir pith yielded adsorption capacities of 159 and 89 mg/g for acid brilliant blue and procion orange, respectively. Mechanisms involving ion exchange and chemisorption might be responsible for the uptake of dyes. (c) 2006 Wiley Periodicals, Inc.