A thermocouple shows a high accuracy in reading temperatures, but it does not always give a true value for the temperature of the leaf against which it is pressed on. Temperature differences between the leaf and the air moving around it cause deviations of the shown temperature from the actual leaf temperature. A method is described to calibrate thermocouples inside gas exchange cuvettes without obstructing the movement of the air around the leaf, so that the heat exchange between thermocouple and air is taken into account. The reading of the leaf temperature in a steady-state porometer was checked by this method at various temperatures of ambient air (T a ) and of the leaf (T 1 ) and was found to give an average value of T a and T 1 . The effect of incorrect estimation of the leaf temperature on computed diffusive resistances on H2O (r w ) and CO2 (r c ) and intercellular CO2 partial pressures (p ic ) is discussed.