With the emergence of wide-spread application of cavity ring-down spectrometers (CRDS) to monitor δ¹³C in atmospheric CO₂ there is a growing need to ensure well calibrated measurements. We characterized a cavity ring-down spectrometer system used for continuous in-situ monitoring of atmospheric ¹³CO₂. We found no concentration dependency of the δ¹³C ratio within the range of 303–437 ppm. We designed a calibration scheme according to the diagnosed instrumental drifts and established a quality assurance protocol. We find that the repeatability of 10 min measurements is 0.25‰ and 0.15‰ for 20 min integrated averages. We found the cross-sensitivity to C₄ in the samples to be 0.42 ± 0.02‰ ppm⁻¹. Our ongoing target measurements yield standard deviations of 0.26–0.28‰ for 10 min averages. We furthermore estimate the reproducibility of the system for ambient air samples from weekly measurements of a long-term target gas to be 0.18‰. We find only a miniscule offset of 0.002 ± 0.025‰ of the CRDS and Environment Canada's isotope ratio mass spectrometer (IRMS) results for four target gases used over the course of one year.