NDIR (nondispersive infrared spectroscopy) allows the continuous analysis of respiratory gases. Due to its high selectivity, simple and robust set up and small footprint, it is also used to support ¹³CO₂-breath tests to assess bacterial growth in the stomach, gut or liver function. CO₂ NDIR signals, however, are biased by oxygen in the gas matrix. This complicates NDIR based breath tests, if the inspired oxygen concentration has to be adjus­ted to the subjects requirements or hyperoxia induced effects were studied. To avoid the oxygen-induced bias, a "dilution" approach was de­veloped; expired gas is mixed with N₂ to lower the oxygen content down to the usual range of 15%-20%. Accuracy and precision were tested using synthetic gas mix­tures with increasing ¹³CO₂ / ¹²CO₂ ratios, either based on synthetic air with about 20 %Vol O₂ or on pure O₂ . For samples with ¹³C values smaller 300 (or ¹³CO₂ / ¹²CO₂ ratios smaller 0.003) the dilution does not significantly increase the bias in the ¹³CO₂ / ¹²CO₂ ratio determination and the within-run imprecision is smaller 1 ¹³C. The practical use of this approach was validated in a pig study using a sepsis model reflecting a clinical situation that requires an increased oxygen concentration for respiration. The N₂-dilution eliminated the high bias in NDIR- measurement thus allowing to determine the impact of oxygenation on glucose oxidation in patients ventilated with increased oxygen.