There is an increasing ecological interest in understanding the gradients in H(2)(18)O enrichment in leaf water (i.e. a Péclet effect), because an appreciation of the significance of the Péclet effect is important for improving our understanding of the mechanistic processes affecting the (18)O composition of leaf water and plant organic material. In data sets where both source water and leaf water (18)O data are available, we can evaluate the potential contribution of a Péclet effect. As an example, we recalculate data published earlier by Roden and Ehleringer (1999, Oecologia 121:467-477) as enrichments in leaf water (Delta(L)) and cellulose (Delta(cell)) above source water. Based on these recalculations, we present support for the relevance of a Péclet effect in leaves. Further, we demonstrate that the subtle variations in Delta(L) and Delta(cell) caused by a Péclet effect may be masked in experimental systems in which variation in the source water oxygen isotope ratio is considerable.