A concept on the adsorption kinetics of surfactants was explored and illustrated with l-octanol and C12E8 recently (Langmuir 1997, 13, 6204): using only a limited range of equilibrium surface tension data to determine the adsorption isotherm can cause a serious mistake on the determination of adsorption kinetics and/or on the evaluation of diffusion coefficient from the dynamic surface tension data. To learn the effect of the intermolecular interaction between adsorbed molecules, a systematic theoretical study is performed for surfactants adsorbing onto a clean planar air-water interface. Data from the theoretical simulation indicate that the stronger are the surfactants interactions (i.e., with a larger absolute K value), the greater are the deviations in apparent diffusivity or in sorption rate constants obtained from a Langmuir analysis. Besides, a larger deviation on diffusivity results at more dilute concentration or when a smaller range of surface tension data are utilized to determine the model constants.