The influence of nitrogen atoms on the adsorption of CO on a Rh(100) single crystal surface has been studied by a combination of experimental techniques: low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and reflection absorption infrared spectroscopy (RAIRS). Dynamic Monte Carlo simulations have been used to model how the nitrogen atoms are distributed over the surface at different coverages. The nitrogen layer consists of small interconnected c(2 × 2)-N/Rh(100) islands with in between the islands well-defined sites for CO to adsorb onto. Two short-range ordered nitrogen geometries were identified with coadsorption sites binding CO less strongly. Nitrogen greatly influences adsorbed CO: a pairwise repulsive CO/N-interaction energy of ¿N-CO = 21 kJ/mol was obtained.