Over the past few years detailed studies of the interaction between alkalimetal adatoms and CO have been made on a wide range of single-crystal substrates using a range of surface science techniques, including vibrational spectroscopy, temperature programmed desorption, LEED, photoemission, NEXAFS and work function changes. In this paper recent experimental work at Liverpool on Cu{110} and Pd{110} is reviewed alongside results from other laboratories on Ru{110}, Fe{111}, Fe{110}, Pt{111}, Pd{111}, Ni{111} and Cu{100}. On all surfaces at room temperature and above a strong interaction is revealed, particularly through increases in desorption temperature and dramatic decreases in C—O stretching frequencies. All the results are critically analysed in terms of various models which have been postulated. Weak interactions of an electrostatic nature are observed between ammonia and CO, in agreement with theoretical calculations; but between CO and K a strong chemical interaction occurs, which may be activated. Strong interactions can result in the production of polymeric CO anions of the form (CnOn)2–. These anions, containing C—C bonds, may be important in the Fischer–Tropsch processes.