We investigated the structure of the water monolayer on an open surface, Cu(110), at low temperature. We found that water adsorbs molecularly, adopting a 2:1 ratio of H-down and H-up configurations. This behavior of water on an open surface is quite different to the behavior on close-packed surfaces, such as Pt(111) and Ru(0001), where water adsorbs primarily H-down, but can be understood on the basis of a range of different water adsorption sites across the observed (7 x 8) unit cell. (c) 2006 Elsevier B.V. All rights reserved.