The sorption of acid dyes from aqueous effluents onto activated carbon has been studied. The effects of initial dye concentration and activated carbon mass on the rate of Acid Blue 80 and Acid Yellow 117 removal have been investigated. Three mass transport models based on film and pore diffusion control have been applied to model the experimental concentration decay curves. The models are compared on the basis of the solid-phase loading capacity using various assumptions since the assignment of an appropriate solid-phase loading has been the subject of several papers on this topic and no comparisons have been provided on the effectiveness of each approach. The equilibrium solid-phase concentration is assumed: (i) incorporating a time-dependent solid-phase concentration Y-e,Y-t,Y- (ii) equal to the intersection point of the equilibrium isotherm and the operating line and (iii) the point on the equilibrium isotherm where the liquid-phase concentration equals the initial concentration in the film-pore diffusion model. (C) 2003 Elsevier Ltd. All rights reserved.