The adsorption of Acid Yellow 99 (AY99) onto dodecylethyldimethylammonium (DEDMA)-sepiolite was investigated in aqueous solution in a batch system with respect to pH, contact time and temperature. The surface modification of DEDMA-sepiolite was examined by the FTIR technique. The experimental data fitted very well with the pseudo-second-order kinetic model with the correlation coefficients above 0.985. The Langmuir, Freundlich and Dubinin-Radushkevich (D-R) adsorption isotherm models agree well with experimental data. The Langmuir-isotherm constant (K-L) was used to evaluate the thermodynamic parameters for the adsorption of AY99 onto DEDMA-sepiolite.