The leaf water potential measured on leaf discs with a Wescor (Spanner-type) psychrometer increases with exposure time of the leaf tissue in the psychrometer chamber. A constant value is obtained after 30 to 45 min. The possibility is investigated that these variations reflect an internal equilibration of water potentials within the leaf disc through water movement from the interior of the tissue where the water potential is relatively high at the time of cutting the disc to the surface where the potential in intact plants is relatively low. The psychrometer registers the water potential at an effective surface of the leaf disc. This hypothesis is supported by substantial potential differences that have been reported in the literature and by the qualitative and quantitative agreement of the equilibration pattern with a theoretical model. Various methods of extraction and measurements yielded comparable values for the osmotic potential component.