For the purpose of description the adsorption process of amylbenzene on a C-8, C-18 and C-30-bonded silica stationary phase with methanol-water (80:20, v/v) as the mobile phase was proposed as a novel adsorption model called the cluster isotherm model. The model assuming possibility of independent adsorption of analyte clusters on the longer C-30 and shorter C-8 chains. The validation of the proposed isotherm was made by comparison of experimental breakthrough profiles obtained for RP-8e and RP-30 columns with theoretical simulation using Transport-Dispersive model.