Oxidation of methionine residues in biopharmaceuticals is a common and often unwanted modification that frequently occurs during their manufacture and storage. It often results in a lack of stability and biological function of the product, necessitating continuous testing for the modification throughout the product shelf life. A major class of biopharmaceutical products are monoclonal antibodies (mAbs), however, techniques for their detailed structural analysis have until recently been limited. Hydrogen/deuterium exchange mass spectrometry (HXMS) has recently been successfully applied to the analysis of mAbs. Here we used HXMS to identify and localise the structural changes that occurred in a mAb (IgG1) after accelerated oxidative stress. Structural alterations in a number of segments of the Fc region were observed and these related to oxidation of methionine residues. These included a large change in the hydrogen exchange profile of residues 247-253 of the heavy chain, while smaller changes in hydrogen exchange profile were identified for peptides that contained residues in the interface of the C(H)2 and C(H)3 domains.