Thermocouple psychrometers are being used increasingly in plant-water relation studies, as they offer a convenient and accurate method of determining the water potential (IY) of plant tissue. Results, however, have not always been satisfactory due to poor techniques and inadequate precautions during sampling. Excessive evaporative losses during the sample excision process can be minimized by using a humidified box in which to conduct the sampling, shading the tissue during excision, and minimizing the time between excision and sealing in the psychrometer chamber. The position sampled on a leaf can affect the results due to the influence of veinal tissue and gradients of IY. Damage to sample sections from bruising or crushing during sampling can also alter the IY. Good sampling techniques and adequate precautions to reduce evaporative losses resulted in accurate and reliable IY measurements of leaves, bolls and subtending bracts under laboratory or field conditions.