Surface superstructures induced by adsorptions of divalent atoms (Ca and Sr) on a Si(111) substrate were investigated by reflection-high-energy electron diffraction (RHEED). After depositing Ca or Sr of 1∼3 monolayers, followed by post heating at temperatures ranging from 400 °C to 900 °C, successive changes in structure were observed due to the coverage decrease by thermal desorption of the adsorbates; 1 × 1, √3 × √3, 2 × 1, 9 × 1, 7 × 1, 5 × 1, 3 × 1, and also complicated undefined superstructures were observed before returning to the clean 7 × 7 structure. While the observed changes were similar to each other between the Ca and Sr cases, there were differences. The 9 × 1 phase was clear for Sr adsorption, while the 7 × 1 was distinct for Ca case. For the Sr case, the superlattice spots of the adjacent phases were coexisted at the intermediate coverages, meaning that the domains of the two phases coexisted separately (phase separation). For Ca case, on the other hand, the superlattice spots continuously shifted in position to those of the next phase with decrease of Ca coverage. This phenomenon was well reproduced by a simple kinematical simulation of diffraction. For example, when the 5 × 1 and 3 × 1 superstructures are mixed microscopically with different ratios, a new superlattice spot appears between the fifth- and third-fractional order spots, and its position shifts depending on the area ratio. Such a microscopic phase mixture is consistent with the previous observations by scanning tunneling microscopy. [DOI: 10.1380/ejssnt.2004.178]