Chitosan membranes were obtained by dissolving the biopolymer in a solution of 5% acetic acid (v/v). Chemical modification of chitosan was done by the addition of thioacetamide in order to obtain a material that absorbed textile dyes more efficiently. The material was analyzed by infrared spectroscopy. The spectra showed that the introduction of thioacetamide to the structure of chitosan did not significantly modify the bands of the biopolymer. The spectra included bands attributed to the C=S bond. The Langmuir isotherm was used to experimentally determine absorption capacity data, and showed that modification of chitosan with thioacetamide caused a significant increase in this parameter. The kinetics of adsorption was studied. It was found that systems that have used chitosan modified with thioacetamide attained equilibrium within 60 minutes of the start of the reaction. The experimental data was modified by use of the kinetic model of pseudo-second-order, and the equilibrium constants were determined.