Cavity-enhanced Raman multigas spectrometry is introduced as a versatile technique for monitoring of (13)CO2 isotope labeling experiments, while simultaneously quantifying the fluxes of O2 and other relevant gases across a wide range of concentrations. The multigas analysis was performed in a closed cycle; no gas was consumed, and the gas composition was not altered by the measurement. Isotope labeling of plant metabolites via photosynthetic uptake of (13)CO2 enables the investigation of resource flows in plants and is now an important tool in ecophysiological studies. In this experiment the (13)C labeling of monoclonal cuttings of Populus trichocarpa was undertaken. The high time resolution of the online multigas analysis allowed precise control of the pulse labeling and was exploited to calculate the kinetics of photosynthetic (13)CO2 uptake and to extrapolate the exact value of the (13)CO2 peak concentration in the labeling chamber. Further, the leaf dark respiration of immature and mature leaves was analyzed. The quantification of the photosynthetic O2 production rate as a byproduct of the (13)CO2 uptake correlated with the amount of available light and the leaf area of the plants in the labeling chamber. The ability to acquire CO2 and O2 respiration rates simultaneously also simplifies the determination of respiratory quotients (rate of O2 uptake compared to CO2 release) and thus indicates the type of combusted substrate. By combining quantification of respiration quotients with the tracing of (13)C in plants, cavity enhanced Raman spectroscopy adds a valuable new tool for studies of metabolism at the organismal to ecosystem scale.