The influence of CO2 transported in the transpiration stream on measurements of leaf photosynthesis and stem respiration was investigated. Measurements were made on trees in a temperate forest in Scotland and in a tropical rain forest in Cameroon, and on shrubs in the Sahelian zone in Niger. A chamber was designed to measure the CO2 partial pressure in the gas phase within the woody stems of trees. High CO2 partial pressures were found, ranging from 3000 to 9200 Pa. Henry's Law was used to estimate the CO2 concentration of xylem sap, assuming that it was in equilibrium with the measured gas phase partial pressures. The transport of CO2 in the xylem sap was calculated by multiplying sap CO2 concentration by transpiration rate. The magnitude of aqueous transport in the studied species ranged from 0.03 to 0.35 μmol CO2 m−2 s−1, representing 0.5 to 7.1% of typical leaf photosynthetic rates. These values strongly depend on sap pH. To examine the influence of aqueous transport of CO2 on stem gas exchange, we made simultaneous measurements of stem CO2 efflux and sap flow on the same stem. After removing the effect of temperature, stem CO2 efflux was positively related to sap flow. The apparent effect on measurements of stem respiration was up to 0.7 μmol m−2 s−1, representing ∼12% of peak stem respiration rates.