Understanding mechanisms for forming protein coronas on nanomaterial surfaces is essential to designing drug delivery systems and designing and interpreting the results of nanomaterial toxicity tests. The study reports the adsorption behavior of a positively charged protein, lysozyme, on cerium dioxide (ceria) nanoparticles with three different surface charges. Adsorption isotherms were modeled with the Toth and Sips equations. Isotherm loading levels were compared to monolayer coverage estimate for 'side-on' and 'end-on' lysozyme orientations as well as random packing (jamming) and maximum packing limits. Evaluation of adsorption site energy distributions (generated using the model coefficients) suggested that the negatively charged ceria surface had a very broad site energy distribution and that its surface heterogeneity controls the adsorption process. By contrast, the adsorption of lysozyme on the positively charged nanoparticles appears to be influenced by lateral effects from adsorbed protein species. The results illustrate the importance of nanoparticle surface chemistry to protein adsorption. The modeling and site energy distribution evaluations may be useful for interpreting the formation of protein coronas on nanoparticles. (C) 2012 Elsevier B. V. All rights reserved.