We use single-mode, optical waveguide lightmode spectroscopy to measure the mass of protein adsorbed at the waveguide/liquid interface at a frequency and precision sufficient to achieve sets of adsorption rate data (d Gamma /dt, where Gamma is the adsorbed protein density) that clearly delineate distinct diffusion- and reaction-limited kinetic regimes. These data suggest that the adsorption can be modeled by combining a particle approach at the surface with a boundary layer approach above the surface. The rate-limiting mechanisms responsible for the observed kinetic regimes appear as "resistors" in our transport/adsorption model.