Dynamics of adsorption of 14 C radiolabeled -lactoglobulin and ␣-lactalbumin at the air-water interface was investigated through the measurement of surface pressure () and surface concentration (⌫) via a radiotracer technique. Adsorption was diffusion controlled at short times, the rates of increase of and ⌫ being lower at longer times because of an energy barrier. At low concentrations, an apparent time lag was observed in the evolution of for -lactoglobulin but not for ␣-lactalbumin which was shown to be due to the nonlinear nature of the -⌫ relationship for the former. The area per molecule of an adsorbed -lactoglobulin during the dynamics of adsorption was smaller than that for spread monolayer since -lactoglobulin was not fully unfolded during adsorption. For ␣-lactalbumin, however, no such difference in the molecular areas for adsorbed and spread monolayer was observed indicating thereby that ␣-lactalbumin unfolded much more rapidly than -lactoglobulin. Evolution of ⌫ for ␣-lactalbumin was found to occur in two steps possibly due to the change in the orientation of the adsorbed protein from a side-on to an end-on orientation. A previously developed mechanistic model (G. Narsimhan and F. Uraizee, Biotechnology Prog. 8, 187 (1992)) was improved to account for the presence of hydrophobic patches on the surface of the protein molecule as well as an adsorbed protein layer at the air-water interface. The model predictions agreed quite well with the experimental evolution of ⌫ for -lactoglobulin and ␣-lactalbumin. The model calculations seem to indicate that ␣-lactalbumin changes its orientation at the airwater interface from side-on to other orientations at higher surface concentrations.