Scanning tunneling microscopy is used to characterize the electronic structure of 1 ML films of CuN on Cu(100). We find that CuN acts as an insulator with a band gap that exceeds 4 eV. Measurements of the tunneling barrier height and image potential states indicate that the CuN work function is ∼0.9 eV larger than bare Cu. This suggests a significant surface dipole, consistent with charge transfer predicted by theory. Our results indicate that CuN films are useful for controlling the electronic coupling between adsorbates and surface electron density on the nanometer scale.