The adsorption of three single-component acid dyes onto activated carbon has been studied using fixed-bed adsorption. Conventional simplified design methods, namely, bed depth service time (BDST) and empty-bed residence time (EBRT) models, were applied to the experimental breakthrough curve data but failed to correlate these data. Consequently, modifications have been developed that enable modified BDST and EBRT models to be applied and to correlate the experimental data very accurately. The modification is based on an expression to correlate the residence time in the adsorption bed with the time-dependent fraction degree of saturation of the bed. This model is particularly suited to predicting the performance of fixed-bed adsorbers when the system requires a long period of time to reach equilibrium or when several fixed-bed adsorbers are used in series.