Leaf temperatures of tropical rain forest plants, weeds, crops, as well as temperate beach and dune plants, were measured by contact with fine ribbon thermocouples and by a non-contact thermal radiation method. With strong solar radiation, non-transpiring leaves (covered with vaseline) in the tropics usually have temperatures of more than 10 °C above air temperature. Temperature differences of up to 20°C were measured. The highest temperature differences leaf-air of temperate plants are only a few degrees centigrade below those of tropical plants. The temperatures of fresh leaves (both tropical and temperate) may vary in full sunshine from somewhat below air temperature to virtually the same temperature as the vaseline covered leaf. The question was asked whether the strong transpiration cooling observed on leaves of Saccharum officinale, Calotropis gigantea and Phragmites communis and the extreme transpiration cooling (13 °C below air temperature) observed by Lange in the Sahara had to be explained by special properties of the leaf or rather by the dryness of the atmosphere. From the temperature measurements on the leaves and the vapour pressure of the air it is possible to calculate a factor n which is the ratio of the total resistance of the leaf to water loss and the external resistance. For weeds and production crops typical values of n are between 3 and 6. Similar values were found for the sclerophyllous Eryngium maritimum and for Aster tripolium. For some species a high n (> 50) seems to be normal. The data indicate that strong transpiration cooling is not due to an abnormally low water loss resistance but to the extreme dryness of the air.