The trace gas glyoxal (CHOCHO) forms from the atmospheric oxidation of hydrocarbons and is a precursor to secondary organic aerosol. We have measured the absorption cross section of disubstituted 13CHO13CHO (13C glyoxal) at moderately high (1 cm-1) optical resolution between 21 280 and 23 260 cm-1 (430-470 nm). The isotopic shifts in the position of absorption features were found to be largest near 455 nm (Δν = 14 cm-1; Δλ = 0.29 nm), whereas no significant shifts were observed near 440 nm (Δν < 0.5 cm-1; Δλ < 0.01 nm). These shifts are used to investigate the selective detection of 12C glyoxal (natural isotope abundance) and 13C glyoxal by in situ cavity enhanced differential optical absorption spectroscopy (CE-DOAS) in a series of sensitivity tests using synthetic spectra, and laboratory measurements of mixtures containing 12C and 13C glyoxal, nitrogen dioxide, and other interfering absorbers. We find the changes in apparent spectral band shapes remain significant at the moderately high optical resolution typical of CE-DOAS (0.55 nm fwhm). CE-DOAS allows for the selective online detection of both isotopes with detection limits of ∼200 pptv (1 pptv = 10-12 volume mixing ratio), and sensitivity toward total glyoxal of few pptv. The 13C absorption cross section is available for download from the Supporting Information.