In the presence of deuterated water, hydrogen atoms with solvent accessibiltiy can exchange with the deuterons. Mass spectrometry can be used to detect the changes in mass corresponding to incorporation of deuterium. This is a reversible process and care must be taken to avoid conditions that promote the back-exchange or scrambling of the position of the exchanged deuterons. Using a refridgerated LC column module and online pepsin digestion, these detrimental proceses can be minimised. Under these conditions exchange at the aminde nitrogens on the protein backbone are probed. The major advantage of this technique over covalent modification is that different deuterium labelling times can be used which enables information on the kinetics of exchange, and thus information on the deuteron's environment, can be probed.