Rationale: Cavity ring-down spectroscopy (CRDS) is becoming increasingly popular for C-13-CO2 analysis of air. However, little is known about the effect of high C-13 abundances on the performance of CRDS. Overlap between (CO2)-C-12 and (CO2)-C-13 spectral lines may adversely affect isotopic-CO2 CRDS measurements of C-13-enriched samples. Resolving this issue is important so that CRDS analysers can be used in CO2 flux studies involving C-13-labelled tracers. Methods: We tested a Picarro G2131-i CRDS isotopic-CO2 gas analyser with specialty gravimetric standards of widely varying C-13 abundance (from natural to 20.1 atom%) and CO2 mole fraction (xCO(2): <0.1 to 2116ppm) in synthetic air. The presence of spectroscopic interference between (CO2)-C-12 and (CO2)-C-13 bands was assessed by analysing errors in measurements of the standards. A multi-component calibration strategy was adopted, incorporating isotope ratio and mole fraction data to ensure accuracy and consistency in corrected values of C-13-CO2, x(12)CO(2), and x(13)CO(2). Results: CRDS measurements of x(13)CO(2) were found to be accurate throughout the tested range (<0.005 to 100ppm). On the other hand, spectral cross-talk in x(12)CO(2) measurements of standards containing elevated levels of (CO2)-C-13 led to inaccuracy in x(12)CO(2), total-xCO(2) (x(12)CO(2)+x(13)CO(2)), and C-13-CO2 data. An empirical relationship for x(12)CO(2) measurements that incorporated the C-13/C-12 isotope ratio (i.e. (CO2)-C-13/(CO2)-C-12, RCO2) as a secondary (non-linear) variable was found to compensate for the perturbations, and enabled accurate instrument calibration for all CO2 compositions covered by our standard gases. Conclusions: (13)C-enrichement in CO2 leads to minor errors in CRDS measurements of x(12)CO(2). We propose an empirical correction for measurements of C-13-enriched CO2 in air by CRDS instruments such as the Picarro G2131-i.