The adsorption of nitric oxide on Cu(100) and Cu(111) surfaces has been studied by combining X-ray and u.v. photoelectron spectroscopy with low energy electron diffraction. At 80 K two molecular states, characterised by N(1s) values of 399.5 and 401 eV, have been assigned to “bent” and “linear” configurations, respectively. The bent form dissociates slowly at 80 K while the linear species desorbs above ? 170 K. Assignments of the “bent” and “linear” forms were facilitated by recourse to the known sterochemistry of metal nitrosyl complexes where a correlation between formal charge on the ligand and sterochemistry has been established. Furthermore the results are compared with the adsorption of NO on both clean nickel and nickel whose surface reactivity has been controlled by pre-exposure to oxygen.