Electrostatic and van der Waals interactions are important in many colloidal phenomena and as such have been studied and described extensively. In a previous paper (Langmuir 1993, 9, 962), we described a model for the adsorption equilibrium constant of proteins at charged surfaces based on these effects, calculated using molecular properties of the proteins. Here, we compare the predictions of this model to experimentally determined equilibrium constants obtained using total internal reflectance fluorescence (TIRF) spectroscopy. With this Surface-sensitive technique, equilibrium constants were obtained over 4 orders of magnitude as the ionic strength was varied over a decade and a half. Lysozyme adsorbed preferentially compared to chymotrypsinogen A at low ionic strengths while the opposite trend was observed at high ionic strengths. The model, in its various degrees of complexity, is able to predict the trend observed in equilibrium constant with varying ionic strength, to predict the values approximately, and to discern differences between the behaviors of the proteins studied. The utility of this model in understanding the interplay between charge and size-dependent forces is demonstrated, as is its potential for predictive calculations.