We have applied the Car-Parrinello method to determine structural and electronic properties of Si(111) in the presence of one monolayer of Na and Li. In both cases we find that the alkali adsorption stabilizes the 1 × 1 structure. In spite of the overall similarity in the chemical bonding, we find that the energitically favored adsorption sites are different. The substrate relaxation upon adsorption is minor in the case of Na and significant in the case of Li, where it also plays a crucial role in determining the equilibrium configuration. In both cases, small distortions in the uppermost Si layers are predicted along the  direction. Both systems turn out to be non-metallic, in agreement with the experimental data for Na. The surface band dispersion is in very good agreement with photoemission data on Si(111): Na and is predicted to be less flat for Si(111): Li. © 1992.