A novel reversible order-order phase transition is revealed by reflection absorption infrared spectroscopy and low-energy electron diffraction studies of a chemisorbed monolayer of CO on Co{10 (1) over bar 0} between 100 and 150 K. The high-temperature phase consists of tilted two-fold bridging CO molecules in the well-established p(2 x 1)g structure, while the low-temperature phase involves the movement of one-third of the CO molecules into three-fold hollow sites to produce a new p(6 x 1)g structure. The temperature-induced site switching is attributed to a vibrational-entropy-driven second-order displacive phase transition. (C) 1997 American Institute of Physics.