The theory of diffusion-controlled adsorption kinetics is developed for adsorption layers of surfactant molecules able to form two-dimensional aggregates. It is shown that the formation of such aggregates within the adsorption layer results in a deceleration of the surface tension decrease and in a reduction of the dynamic surface pressure when the system is far from the equilibrium state. If no aggregation of adsorbed molecules is considered, then any decrease in adsorption rate is ascribed usually to the existence of an adsorption barrier. However, assuming this two-dimensional aggregation, the diffusion model provides a much better description of phenomena that actually take place. A good correspondence is found between the experimental dynamic surface tensions and theoretically calculated dependencies for the solutions of l-decanol for a mean aggregation number of 2.5 within the monolayer. The same aggregation number was estimated for decanol equilibrium monolayers.