The coadsorption of NO and NH3 on Cu(111) was investigated by means of LEED, HREELS, and TPD. A new ordered (2 x 2) structure was found after annealing to 180 K NO-NH3 coadlayers prepared from various relative exposures of NO and NH3 at 100 K. A N2O desorption peak from the coadlayer appeared at 220 K, which was 70 K higher than the desorption peak From a pure NO layer. A new NH3 desorption peak also appeared al 220 K, while there was only a broad peak centered at 150 K for a pure ammonia adlayer. These results suggest that the coadlayer is more stabilized than the pure layers owing to the attractive interaction between NO(a) and NH3(a). While NO in pure NO layers adsorbs on atop, bridge, and 3-fold hollow sites, it was found that NO in the (2 x 2) structure occupied only 3-fold hollow sites in a linear configuration and the frequency of v(N-O) shifted downward from that in pure NO layers. This change in adsorption state of NO correlates with enhanced occupation of the antibonding 2 pi* orbital of NO due to charge transfer from NH3 through the substrate, leading to more attractive interaction. A real space model for the (2 x 2) structure was proposed on the basis of the results of TPD and HREELS.