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 (CHOCHO)-C-13-C-13 (C-13 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 (Delta nu = 14 cm(-1); Delta lambda = 0.29 nm), whereas no significant shifts were observed near 440 nm (Delta nu < 0.5 cm(-1); Delta lambda < 0.01 nm). These shifts are used to investigate the selective detection of C-12 glyoxal (natural isotope abundance) and C-13 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 C-12 and C-13 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 similar to 200 pptv (1 pptv 10(-12) volume mixing ratio), and sensitivity toward total glyoxal of few pptv. The C-13 absorption cross section is available for download from the Supporting Information.