In this paper we investigate the adsorption thermodynamics of dilute solutes adsorbing from high pressure supercritical fluid phases. Both pure and mixed solvent phases consisting of carbon dioxide-ethane mixtures have been studied with the data presented covering a wide range of thermodynamic conditions including those in the system’s near-critical regime. Large polycyclic organic species have been used as solute species since these are representative of compounds of most practical interest in the field. The analyses of the data have used various approaches for representing and predicting adsorption equilibrium constants and other thermodynamic properties in these systems. In the first approach we have adapted the Bragg-Williams lattice model for the stationary phase properties in conjunction with an equation of state for the fluid phase. In a subsequent analysis we have investigated the use of the Henry’s law limit for the solute’s stationary phase properties. This latter approach shows the potential for leading to very useful methods for predicting stationary phase properties in multicomponent systems of this kind and its efficacy for these purposes has been evaluated with data from two different systems. In addition to adsorption coefficient measurements, solute heats of adsorption can be found using the results presented. In the toluene-carbon dioxide system the results of this study compare favorably with those reported by Recasens et al. (1993).