The RuO2(100) surface exposes regions with (1 x 1) and c(2 x 2) symmetry. The (1 x 1) phase is very active in oxidizing CO, while the c(2 x 2) phase is inactive because CO does not adsorb on the c(2 x 2) phase above 100 K. This experimental finding suggests the absence of undercoordinated 1f-cus Ru sites on the RuO2-(100)-c(2 x 2) surface. Low-energy electron diffraction indicates that the c(2 x 2) phase is a reconstructed (1 x 1) phase. The catalytically inactive c(2 x 2) phase plays an important role in the deactivation of Ru-based catalysts.