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The theoretical basis for the link between the leaf exchange of carbonyl sulfide (COS), carbon dioxide (CO₂) and water vapour (H₂O) and the assumptions that need to be made in order to use COS as a tracer for canopy net photosynthesis, transpiration and stomatal conductance, are reviewed. The ratios of COS to CO₂ and H₂O deposition velocities used to this end are shown to vary with the ratio of the internal to ambient CO₂ and H₂O mole fractions and the relative limitations by boundary layer, stomatal and internal conductance for COS. It is suggested that these deposition velocity ratios exhibit considerable variability, a finding that challenges current parameterizations, which treat these as vegetation‐specific constants. COS is shown to represent a better tracer for CO₂ than H₂O. Using COS as a tracer for stomatal conductance is hampered by our present poor understanding of the leaf internal conductance to …