Using in situ polarization modulation infrared reflection absorption spectroscopy and density functional theory calculations, a new high-coverage monomeric NO adsorption state on Pd(111) was observed and proposed to have a (3 x 3)-7NO structure. Formation of this high coverage NO phase was found to take place only at elevated pressure and temperature conditions showing that some of the accessible thermodynamic equilibrium states at elevated temperatures and pressures are thermodynamically unfavorable or kinetically hindered at lower temperatures and pressures. Our results emphasize the danger of extrapolating results from traditional surface science experiments performed under ultrahigh vacuum to elevated temperature and pressure conditions encountered in heterogeneous catalysis.