Reduction, surface oxidation and re-reduction of a copper-on-silica catalyst has been investigated using hydrogen, carbon monoxide and nitrous oxide in a flow system with mass spectrometric determination. Hydrogen is more effective than carbon monoxide for the initial reduction of the catalyst as prepared in oxide form and the copper content can be accurately determined from the total gas consumption in both cases. However, carbon monoxide can quantitatively remove surface oxygen, deposited from nitrous oxide, at a temperature of 333 K, whereas the same process in hydrogen peaks at 383 K. A method for determining the quantity of surface copper by sequential N2O/CO cycles under isothermal conditions has been established.