Structural and reactivity aspects of Cu(110)-Cs and Cu(110)-Cs/O overlayers have been investigated by XPS and STM. The development of cesium-induced structures at a Cu(110)-Cs overlayer has been followed as a function of cesium coverage at 295 K and the reactivity of the overlayer first to oxygen and subsequently to ammonia and carbon dioxide studied. For cesium concentrations up to 1.3 x 10(14) cm(-2) an incommensurate pseudo square structure is observed which is in registry with the copper substrate in the <100> direction. This coexists, at 1.3 x 10(14) Cs adatoms cm(-2), with a structure consisting of <1 (1) over bar0>-orientated rows which are not atomically resolved but have a spacing in the <100> direction of 1.1 nm. At a cesium concentration of 1.5 x 10(14) cm(-2), patches of structure with an inter-row spacing of 0.7 nm are present, and at a cesium concentration of 1.9 x 10(14) cm(-2), only the latter spacing is observed. An increase in cesium concentration to 2.1 x 10(14) cm(-2) results in an increase in the inter-row spacing to 1.1 nm. Exposure of cesium-modified surfaces to oxygen results in the development of new terraces superimposed upon the cesium-modified surface with the latter structurally unchanged by the formation of the oxygen adlayer, although there is a change in the XP binding energy of the Cs(3d) peaks. The new terraces consist of <100>-orientated chains similar to those observed at unmodified Cu<1 (1) over bar0> surfaces; however, at low cesium concentrations these chains are arranged in (3 x 1) as well as (2 x 1) domains and at high cesium concentrations a c(6 x 2)O structure develops with an exposure of oxygen as low as 10 L. The adsorption of cesium at partially preoxidized surfaces results in the formation of a c(2 x 4) structure and also structures which have no simple relationship with the substrate lattice and which we assign to strained Cu-0 (2 x 1) structures. At a surface with a complete monolayer of oxygen present the cesium forms <1 (1) over bar0> chains with a minimum interchain spacing of 0.5 nm and alternate chains showing well-resolved maxima with a spacing of 0.5 nm. The adatoms in between the chains are less well-defined suggesting a degree of mobility in the <1 (1) over bar0> direction. In comparison with oxygen at unmodified Cu(110) surfaces, oxygen states at the cesium-modified surface are unreactive to ammonia at 295 K, reaction only occurring after exposures of 300 L at 490 K. However, the cesium surface is reactive to carbon dioxide chemisorption at 295 K resulting in a surface carbonate with extensive migration of cesium and the emergence of areas of the underlying copper surface.