LEED studies reveal that the clean Mo(100) surface reconstruction is modified by small amounts of adsorbed K or Na in such a way that the long-range order is maintained but the surface periodicity changes continuously with coverage. The long-range order is also maintained with the adsorption of small amounts of Li, but no periodicity changes are observed. These findings are discussed in relation to driving mechanisms proposed for the clean reconstruction. Although the changes might be attributed to charge-density-wave formation in a qualitative sense, this mechanism appears unable to provide an explanation for the detailed behavior. On the other hand, it appears possible to describe the observed sequence of structures in terms of a local bonding model that includes alkali atom repulsions.