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　O-18-labeling　method　utilizing　microwave-assisted　acid　hydrolysis　was　developed.　After　being　dissolved　in　O-16/O-18　(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　O-18　within　less　than　1.5　min.　The　rate　of　back　exchange　was　so　slow　that　no　detectable　back　exchange　could　be　found　during　the　HPLC　run.