The high-coverage structures of CO on Co{10 (1) over bar 0} have been investigated in detail using reflection-absorption infrared spectroscopy (RAIRS) and low-energy electron diffraction (LEED) over the temperature range from 100 to 250 K. This has revealed a curious anomaly. As the coverage is increased above 0.5 monolayer (ML) at temperatures below 250 K, a p(2x1) phase, with atop CO, is incompletely converted to a p(2x1)g phase with a local coverage of 1 ML and CO in bridge sites. At temperatures below 180 K, one-third of the surface is converted into the energetically most stable structure, c(2x6), with CO in two types of bridge sites, local coverage 1.17 ML, and the remaining two-thirds remains in the p(2x1)g phase. On cooling to 100 K, the stable c(2x6) phase is unchanged, still occupying one-third of the surface, and the p(2x1)g phase is transformed to a p(6x1)g phase, driven by vibrational entropy. At these low temperatures, this phase transition occurs between two ordered phases that are both metastable with respect to the high-coverage c(2x6) phase. This is attributed to kinetic constraints within the close-packed adsorbed overlayer associated with frozen-in antiphase domains.