The role of alkali promoters in Fischer-Tropsch synthesis was investigated in terms of restricted adsorption/desorption mobility of hydrogen on the metal surface. A Fischer-Tropsch synthesis surface reaction model postulated by Kellner and Bell (J. Catal. 1981, 70, 418) was modified for the removal of surface oxygen steps, and expressions were derived for methane formation rates and chain growth probabilities. The resulting expressions were analyzed in terms of decreased hydrogen adsorption equilibrium constant preexponentials upon alkali promotion. The model predictions were consistent with the experimental data, justifying the role of alkali promoters as modifiers of adsorption/desorption mobility of hydrogen while not interfering with the electronic structure of the metal catalyst.