Protein adsorption is complicated by the many contributions to the overall thermodynamics, and as a result there is a need for mechanistic models in interpreting experimental data. We focus on the electrostatic contribution to the adsorption thermodynamics and, specifically, on calculating the relative contributions of enthalpic and entropic driving forces to the electrostatic component of the free energy of adsorption. The model used is a colloidal one in which the protein, modeled as a sphere carrying a point charge at its center, interacts with a planar adsorbent surface of opposite charge as well as with neighboring adsorbent molecules. It is found that the entropic contribution dominates the protein-surface interaction, which is generally but not uniformly attractive; this contribution reflects the liberation of counterions from the double layers adjacent to the protein and adsorbent surfaces, Protein-protein interactions are found to be generally repulsive but weak, with energetic and entropic contributions similar in magnitude. The dominance of entropic effects is consistent with several reported calorimetric measurements, but additional effects, such as the release of water molecules from the interacting surfaces, confound attempts to compare theory and experiment directly.