Linking measurements of carbonyl sulfide (COS) with those of carbon dioxide (CO2) has a potential in providing a powerful tracer of gross CO2 fluxes between the atmosphere and land plants, a critical element in understanding the response of the land biosphere to global change. A new application of online COS, CO2 and water vapor measurements based on newly designed mid-infrared (IR) dual quantum cascade laser (QCL) spectrometer measures COS and CO2 (at 2056 cm−1) and water vapor (at 2190 cm−1), with detectors cooled thermoelectrically (at −43 °C) or with liquid nitrogen (−197 °C) for improved precision. Using the cryogenic detectors with averaging time of 1 s, precision was 50 pmol mol−1, 0.4 μmol mol−1 and 0.01 mmol mol−1 for COS, CO2 and water vapor, respectively (14, 0.2 and 0.003, respectively, for 60 s averaging time). We measured COS concentrations in ambient air, and changes in the rates of COS, CO2 and water vapor exchange of attached leaves in response to changes in light intensity and ambient COS concentrations. The results were consistent with those of nononline gas chromatography–mass spectrometry for COS and IR gas analyzer for CO2 and water vapor, with a high linear correlation for a broad range of concentrations (R2= 0.85 for COS and R2= 1.00 for CO2 and water vapor). The new methodology opens the way for lab and field explorations of COS fluxes as a powerful new tracer for CO2 exchange in the land biosphere.