The adsorption of human serum albumin (HSA) on the air/water interface was investigated by means of the sessile bubble technique. The surface tension as a function of time was measured analyzing the shape of axisymmetric bubbles by computer-controlled video image analysis. Surface pressure-time data was converted to surface concentration-time curves by means of area-pressure data obtained by a Langmuir surface balance from spread films. Adsorption was measured at HSA concentrations from 10(-4) to 1% and at pH 2.2, 4.0, 5.8, 7.5, and 10.0. Adsorption up to the initial surface concentration of 0.5-1.0 mg m(-2) was found to be essentially irreversible and controlled by a combination of diffusion and convection, while further adsorption was activation controlled with an activation energy term proportional to the surface concentration and one term independent of the surface concentration. The pH dependency of the adsorption rate was relatively small.