The preparation of poly(methacrylic acid)-modified chitosan microspheres and its application for the removal of cationic dyes, methylene blue (MB) and malachite green (MG), in aqueous solution in a batch system were described. The modified chitosan was characterized using SEM, FTIR, and XPS analyses. The effects of the pH of the solution, contact time, and initial dye concentration were studied. The adsorption capacities of the microspheres for the two cationic dyes increased significantly after the modification because a large number of carboxyl groups were introduced. The equilibrium process was better described by the Langmuir than the Freundlich isotherm. According to the Langmuir equation, the maximum adsorption capacities were 1000.0 and 523.6 mg g(-1) for MB and MG, respectively. Kinetic studies showed better correlation coefficients for a pseudo-second-order kinetic model, confirming that the sorption rate was controlled by a chemisorption process. POLYM. ENG. SCI., 49:272-280, 2009. (C) 2008 Society of Plastics Engineers