Protein oxidation by reactive oxygen species has been associated with aging and neurodegenerative disorders, and histidine is one of the major oxidation targets due to its metal-chelating property and susceptibility to metal-catalyzed oxidation. 2-Oxohistidine, the major product of histidine oxidation, has been recently identified as a stable marker of oxidative damage in biological systems, but its biophysical and biochemical properties are understudied, partly because of difficulties in its chemical synthesis. We developed an efficient method to generate a 2-oxohistidine side chain using metal-catalyzed oxidation, applicable to both monomers and peptides. By optimizing reagent ratios and pH buffering in Cu(2+) /ascorbate/O2 reaction system, we improved the yield more than tenfold compared to reported conditions, which allowed us to obtain homogeneously modified 2-oxohisidine peptides for further studies. Analysis of 2-oxohistidine-containing model peptides by liquid chromatography-tandem mass spectrometry demonstrated increased retention time in reverse-phase chromatography and general stability of 2-oxohistidine under electrospray ionization and collision-induced dissociation. Thus, large-scale analysis of 2-oxohistidine-modified proteome should be feasible using shotgun protein mass spectrometry, and we were able to observe such peptides in proteomics datasets. The feasibility of acquiring purified peptide probes and peptide antigens containing 2-oxohistidine will help advance the study of this non-enzymatic posttranslational modification.