The thermocouple psychrometer developed by Richards and Ogata (1) measures the relative vapor pressure of water in air near saturation with great precision. Measurement depends on a temperature difference between wet and dry junctions of a sensitive thermocouple. Richards and Ogata (2) used the thermocouple psychrometer to measure the relative pressure depression of water vapor in air that was equilibrated with soils. As early as 1958 they also measured the relative pressure depression of water vapor in air equilibrated over excised leaves of broad bean and ivy3. The measurement, however, is more difficult for leaves than for soil. Plant leaves have a special layer (epidermis plus cuticle) impeding exchange of water vapor between the tissue within the leaf and the external air. Leaves also have a much smaller volume of water available for vapor equilibration with air in the sample chamber. Therefore, small leaks of water vapor from the sample chamber that have little or no effect on measurements of water relationships of soils may cause large errors in measurements of water relationships in plant leaves. In this paper is described techniques to increase the accuracy of measuring the energy status of water in excised plant leaves with the thermocouple psychrometer.