The interaction of alkali metal vapors with the surface of MgO has been studied by EPR spectroscopy. After the previously described initial step of interaction leading to the formation of surface F-s(+) and F-s color centers, higher amounts of metal vapor generate new paramagnetic centers which are characterized by a distinctive hyperfine structure. Two types of centers have been identified. The first one, found on Na, K, and Rb doped MgO, contains a single alkali metal atom which shares its valence (unpaired) electron with a suitable site on the MgO matrix. The spin density on the metal atom (percent s character) varies from about 0.2 (for Na) to about 0.5 (for Rb). A small but significant contribution of the metal p orbitals in the center is observed in terms of spectral anisotropy. The second type of center (observed for Li, Na, and K) contains three alkali atoms. Similar to the previous case, the unpaired electron density is not totally confined on the alkali cluster but is partially delocalized toward the matrix (especially for Li and Na). A model which gives a common explanation for the observed results, and which also incorporates the known matrix morphology, is discussed in detail. The observed phenomena are compared with similar examples, including the interaction of alkali metals with zeolites and the interaction of alkali metals with liquid ammonia or similar solvents.