The kinetics of adsorption of lysozyme and bovine serum albumin (BSA) at the air-water interface from binary bulk mixtures have been studied. An increase in BSA concentration in bulk phase caused a decrease in the extent of adsorption of lysozyme and an exponential decrease in its apparent diffusion coefficient; the lag time for its adsorption, however, decreased progressively with increasing bulk concentration of BSA. The ratio of BSA to lysozyme in the mixed monolayer increased with increasing ratio of BSA to lysozyme in the bulk phase. However, the unit cell dimensions occupied by BSA and lysozyme in the mixed monolayer were the same as those in single-component monolayers, suggesting that both BSA and lysozyme adsorbed independently, and the thermodynamic state of one protein was not modified by the other protein at the interface. Analysis of variations in interfacial composition of mixed monolayers formed at various bulk concentration ratios of the proteins indicated that adsorption of these proteins from the binary bulk phase did not follow a Langmuir-type competitive adsorption mechanism. Sequential adsorption experiments showed that although bulk phase BSA abruptly stopped adsorption of lysozyme, it could not displace lysozyme already adsorbed at the interface. In contrast, bulk phase lysozyme could neither stop adsorption of BSA nor displace adsorbed BSA. Critical analysis of these results indicated that adsorption of BSA and lysozyme from binary mixtures followed a kinetically controlled, noncompetitive adsorption mechanism. The interfacial composition of the mixed protein layer was primarily, if not solely, determined by the rate of arrival of each protein at the interface and unoccupied unit cells available at the interface at the time of arrival.