The structural and electrostatic properties of two similar-sized globular proteins, hen egg-white lysozyme and bovine milk -lactalbumin, adsorbed to microspheres of a negatively charged polystyrene latex, are determined from adsorption isotherms, plateau adsorption values, and isothermal titration microcalorimetry data. Particular attention is given to the application of proton-titration techniques for which a new robust protocol is presented which accurately predicts the contributions of the aqueous solution (i.e., the blank) to the overall titration behavior of the protein-containing sample. Plateau values for the adsorbed proteins show a complex dependence on pH with a maximum occurring at the isoelectric point of the protein/sorbent complex. Adsorbed-state titration curves differ markedly from the corresponding dissolved-state curves; the data suggest that -lactalbumin undergoes substantial changes in secondary and tertiary structure upon adsorption. In the adsorbed state, alpha-lactalbumin does not exhibit the low-pH native-to-apo-state transition which promotes release of the Ca2+ ion from the ion-binding pocket. Average p Ka data for the two proteins suggest that a fraction of the charged carboxyl groups are in close proximity to the sorbent surface and the adsorbed protein is largely denatured. The data also indicate that protein adsorption is driven by a synergistic combination of sorbent dehydration, protein structural changes, and charge redistribution.