Knowledge of thermodynamic functions describing adsorption isotherms is of much importance for the design and optimization of adsorption processes. This work deals with the investigation of the adsorption behavior under the influence of a modifier. The experimental setup consisted of two supercritical fluid chromatography units. Adsorption isotherms of alpha- and delta-tocopherol were determined at a pressure of 20 MPa and a temperature of 313 K. The stationary phase was an unmodified silica gel, and supercritical carbon dioxide was used as the mobile phase. The modifier 2-propanol was added to the mobile phase in contents of 4.6, 5.5, and 6.5% mass fraction. The experimental results of pure substances were described with cubic Hill isotherms. The adsorption equilibria of the mixture were predicted with the ideal adsorbed solution theory and verified with experimentally determined data.