Surface adatom modifiers that act as promoters or poisons on transition metal catalysts can have a large influence on adsorption kinetics along with their influence on adsorption energetics. Accounting for this effect requires recognition of the role of a new kind of intrinsic precursor -the modifier precursor state. A mathematical description of the adsorption kinetics of molecules on chemically modified or bimetallic surfaces incorporating this state has been derived based on Kisliuk’s statistical model [P. Kisliuk, J. Phys. Chem. Solids 3, 95 (1957)]. The resulting equation can be used to explain satisfactorily data on CO adsorption kinetics on chemically modified (Cl-, P-, S-, H-, O-, and K-precovered) Pt(111) surfaces. This concept should have broad implications for many systems in heterogeneous catalysis, such as bimetallic and alloy surfaces and coadsorbed layers, and for the multicomponent surfaces that are involved in materials growth and processing.