The behavior of CO phases on Rh(110) has been determined under static CO pressures. Three ordered phases are observed as a function of decreasing temperature under equilibrium conditions. Starting from a disordered phase, these are in sequence a c(2×2), a ‘‘(4×2)’’, and a (2×1)p2mg. In addition, a ‘‘(5×2)’’ phase forms irreversibly just prior to the (2×1)p2mg on cooling in low CO pressures (less than 10−5 Pa). Formation of the (2×1)p2mg phase is found to be activated, which may indicate that surface reconstruction is involved.