Supercritical adsorption of carbon dioxide on 13X zeolite pellets and on silica gel is studied experimentally and theoretically. A gravimetric apparatus with provision for in situ density measurement is used for the measurement of excess adsorption isotherms. The volume of the adsorbent and the other solid parts in the measuring cell, which in turn affects the accuracy of the excess isotherms, is estimated by accounting for helium adsorption on the adsorbent. A model based on lattice density functional theory is introduced and used to analyze the effect of geometric confinement on excess adsorption isotherms under supercritical conditions. This is then used to describe adsorption on adsorbents with different pore size distributions, in particular for the systems that have been experimentally studied. The experimental data and the model results are compared, and their satisfactory agreement is discussed. Features such as "bumps" on the descending part of the excess adsorption isotherms in the CO2-13X zeolite system are discussed and explained using the model.