The adsorption, desorption, and surface structural chemistry of CO on Pt(110) have been studied by low energy electron diffraction, Auger spectroscopy, work function measurement, and flash desorption. Chemisorption induces a (1 × 2)→(1 × 1) change of periodicity in the clean reconstructed surface; the adsorption rate follows precursor state kinetics. It is shown that the diffraction and desorption data are consistent with the presence of two physically distinct kinds of CO in the adsorption layer. These species, denoted αCO and βCO, have adsorption energies of ∼105 and ∼129 kJ mol^–1 respectively. βCO predominates under all conditions, and at high coverages ordering of the β state takes place with the formation of a structure which belongs to the space group plgl. The structure of this ordered phase is examined in some detail, and a bonding model for CO on Pt(110) is proposed. This model is shown to be consistent with the presence of the observed α and β states, the facile (1 × 2)→(1 × 1) transformation of the Pt surface, and the formation of the plgl phase at high coverage.