The potential of a continuous wave cavity ringdown spectrometer for monitoring the isotope ratio 13CO₂/12CO₂ and the partial pressure pCO₂ of CO₂ dissolved in water was thoroughly analyzed by quantitative measurements. Running calibration gas standards under typical operation conditions, a elative accuracy of D(d13C[CO₂]) = ±0.1‰ with 120 min averaging time has been demonstrated. Absolute uncertainties were determined to be D(d13C[CO₂]) = ±0.2‰ and D(xCO₂) = ±0.5 ppmv. No principle problems were encountered when using the instrument in combination with a water-air quilibration setup. By contrast, when performing measurements of CO₂ in gas matrices with a composition different from that of ambient air, pressure broadening linewidth effects induced significant errors in both d13C(CO₂) and xCO₂ values. These effects, which compromise the accessible accuracy in environmental studies, can be quantitatively taken into account by using a spectroscopically based correction procedure. Relying on linewidth analysis, the instrument was shown to be capable of continuous and simultaneous measurement of d13C(CO₂), pCO₂, as well as water content and O2 supersaturation, and thus holds the potential for online monitoring of these quantities aboard research vessels.