The iron—constantan thermocouple contact thermometer here described is mounted on spring clamp and can be quickly attached to the leaf; a large number of determinations can be made in a short time. The thermocouple junction equilibrates very rapidly (1.5 sec), has a low heat capacity, and is spring loaded. The performance of the contact thermometer compares well with that of a thermocouple inserted within the leaf mesohyll and with measurements made with an infrared radiation thermometer.