Technical Abstract: Most methods of measuring the diffusive conductance to water vapor of individual plant leaves potentially change the leaf environment by enclosing part of the leaf in order to measure the rate of water vapor exchange and the driving force for that exchange. Some types of porometers rely on the assumption that stomatal responses to the altered environment lag the measurement interval, but do not record the prior environmental conditions. Here I combined leaf temperature measurements with environmental measurements in an energy balance approach to estimate stomatal and boundary layer conductances of upper canopy leaves of three crop species. Stomatal conductances measured with this method were highly correlated with measurements obtained with a steady-state portable photosynthesis system. However, the photosynthesis system often reduced the leaf to air water vapor pressure difference during the measurements sufficiently to increase stomatal conductance. The method provides a new way to simultaneously determine the natural environmental conditions and stomatal and boundary layer conductances of leaves.