In the presence of gaseous NO, Ag catalysts promoted with alkali and chlorine can exhibit extraordinarily high selectivity for ethylene epoxidation. Coordinated UPS/XPS/AES/TDS observations and kinetic experiments with single-crystal model systems and analogous practical catalysts have shown that under appropriate conditions, monolayer films of adsorbed alkali nitrate/nitrite are formed on the Ag surface; these become unstable at ∼ 500 K - the temperature required for ethylene epoxidation. A surface alkali nitrate is identified as the likely catalytically active species on the metal surface. CO2 chemisorption leads to competition for surface alkali: nitroxy-to-carbonate conversion appears to be involved. Experiments on the dispersed catalysts show that there is spillover of K from Ag to Al2O3 with formation of a new alkali state which is present under reaction conditions. This is in line with the behaviour of model Ag(110)/Al2O3/K systems which show a strong nteraction between K and Al2O3.