Abstract CO2 exchange rate (CER), stomatal conductance (Cs), and transpiration in mature attached leaves of ‘Valencia’ orange [Citrus sinensis (L.) Osbeck] were determined outdoors from predawn to 1100 EDT. The maximum values of CER and Cs, which were about 7 μmol CO2·s−1m−2 and 0.27 cm·s−1, respectively, at solar photosynthetic photon flux density (PPFD) of 500-700 μmol·s−1m−2, remained at these levels as PPFD, temperature, and vapor pressure deficit (VPD) between leaf and air continued to increase. Transpiration rates, with maximum values ranging from 1 to 4 mmol H2O·s−1m−2, increased throughout the measurement periods of the morning as leaf-air VPD increased. Thus, photosynthetic water use efficiency decreased with increasing VPD.