The adsorption of medium molecular weight poly(methyl methacrylate) (PMMA) from an apolar solvent onto oxidized aluminum and silicon substrates has been studied by monochromated X-ray photoelectron spectroscopy (XPS). By recording the capacity of the solid surface for the polymer, by XPS, as a function of solution concentration, it has been possible to construct adsorption isotherms for the two systems under study. At low solution concentration, these isotherms are shown to conform to Langmuir adsorption and provide similar values of monolayer coverage, Γm, but significantly different values of the constant term, b, in the Langmuir equation were determined. At higher solution concentration, the adsorption isotherm was different due to an increase in Γm and a dramatic decrease in the constant b. Such observations are thought to result from a change in conformation of the adsorbed molecule at higher solution concentrations. Such effects are not associated with changes of conformation in solution but reflect the adsorption of a more compact molecule, which invariably forms fewer bonds with the substrate. The nature of the adsorption sites present in the oxidized metal substrates and their interactions with PMMA and conformation of the adsorbed molecules were interpreted from the high-resolution XPS spectra. These results were used to interpret the adsorption isotherms in terms of the acido-basic properties of the polymer and substrates, entropy of polymer solutions, and conformation of adsorbed molecules.