We present a molecular theory for multicomponent adsorption kinetics which assumes that the transient configurations of adsorbed particles can be approximated by the corresponding equilibrium configurations at the same coverage. Rate equations are then derived from a scaled particle theory (SPT) equation of state for a two-dimensional fluid mixture of hard convex particles. The predictions of the theory are in excellent agreement with the results of a computer simulation of the competitive adsorption of two species of disks of different diameters.