Oxygen　exchange　may　occur　at　carboxyl　groups　catalyzed　by　acid.　The　reaction,　however,　takes　at　least　several　days　at　room　temperature.　The　long-time　exchanging　reaction　often　prevents　its　application　from　protein　analysis.　In　this　study,　an　18O-labeling　method　utilizing　microwave-assisted　acid　hydrolysis　was　developed.　After　being　dissolved　in　16O/18O　(1:1)　water　containing　2.5%　formic　acid,　protein　samples　were　exposed　to　microwave　irradiation.　LC-MS/MS　analysis　of　the　resulted　peptide　mixtures　indicated　that　oxygen　in　the　carboxyl　groups　from　glutamic　acid,　aspartic　acid,　and　the　C-terminal　residues　could　be　efficiently　exchanged　with　18O　within　less　than　15　min.　The　rate　of　back　exchange　was　so　slow　that　no　detectable　back　exchange　could　be　found　during　the　HPLC　run.　©　2010　American　Chemical　Society.