In order to design and to optimise preparative separations, the knowledge of the underlying thermodynamic functions, i.e., the adsorption isotherms, is the most essential information. Usually these functions cannot be predicted and various methods have been suggested to determine them experimentally. In particular, dynamic methods are attractive regarding time requirements and reliability. Frontal analysis (FA) is frequently applied to measure single solute isotherms. The theoretical background of this method is offered by the classical equilibrium theory of adsorption. Although this theory also explains the shape of multicomponent breakthrough curves, FA is only seldom applied to determine competitive isotherms. In this work FA was used to measure adsorption isotherms of three components as single solutes and in binary and ternary mixtures in a reversed-phase system. The obtained equilibrium data were correlated using the competitive Langmuir isotherm equation, a bi-Langmuir model, the ideal adsorbed solution theory and the real adsorbed solution theory. No substantial improvement of the predictions was achieved using the more complex models instead of the most simple Langmuir model.