A theoretical estimate of the H/D exchange rate for an open trough (e.g. D2O in ambient atmosphere, 50% relative humidity) yields an amount of exchanged liquid of the order of 10^-2 g after 800 minutes for a standard ISIS trough. Recent pilot experiments however show a much greater volume exchange (~10g/800 minutes). We would like to perform a quantitative study of H/D exchange at the air-water interface to determine the rates of exchange of D2O vapour into null-reflecting water for different conditions (surface-to-volume, humidity). Furthermore we intend to study the potential formation of a D2O rich layer at the vapour/liquid interface.We expect that the results will help with future planning of reflectivity experiments involving large are open troughs and will reduce the potential for misinterpretation of data. It will also help further the understanding of this fundamental process.