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.