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Isoaspartic acid (isoAsp) is a common protein modification that spontaneously arises from asparagine or aspartic acid and has been linked to various diseases and health conditions. However, current methods for identifying isoAsp sites in proteins often suffer from ambiguity and have not gained widespread adoption. We developed a novel method that exclusively labels isoAsp with deuterium. This method capitalizes on the unique structural characteristics of isoAsp residues, which possess a free α-carboxyl group and can form an oxazolone ring. Once the oxazolone ring forms, it facilitates racemization at the C α -position, incorporating a deuteron from a D 2 O solvent. The sites of deuterium-incorporated isoAsp in proteins can be unequivocally determined by comparing the precursor and product ion masses of the peptides from proteins reacted in H 2 O and D 2 O. The effectiveness of this method has been demonstrated through its application to model proteins lysozyme and rituximab. Furthermore, we have confirmed that the isoAsp deuterium-labeling reaction efficiently labels both l- and d-isoAsp without distinction, as well as isoglutamic acid (isoGlu), for which no effective detection methods currently exist.