A fixed bed adsorption study has been performed to investigate the effect of S-shaped (Type V) equilibrium isotherm on the breakthrough patterns. The adsorption breakthrough data of benzene vapours on dealuminated zeolite Y were collected by using a lab-scale fixed bed apparatus. It was found that the adsorption system, at influent concentrations from 100 to 5,000 ppm at 303 K, yields breakthrough curves of unusual shapes. A mathematical model was applied to predict the complex breakthrough patterns at various conditions. The experimental and modelling results suggest that the unusual breakthrough curve is due to the basic shape of the equilibrium isotherm.