Tree-ring d13Cis often interpreted in terms of intrinsic wateruse efficiency (WUE) using a carbon isotope discrimination model established at the leaf level. We examined whether intra-ring d13C could be used to assess variations in intrinsic WUE (Wg, the ratio of carbon assimilation and stomatal conductance to water) and variations in ecosystem WUE (Wt, the ratio of C assimilation and transpiration) at a seasonal scale. Intra-ring d13C was measured in 30- to 60-mmthick slices in eight oak trees (Quercus petraea). CanopyWg was simulated using a physiologically process-based model. High between-tree variability was observed in the seasonal variations of intra-ring d13C. Six trees showed significant positive correlations between Wg calculated from intra-ring d13C and canopy Wg averaged over several days during latewood formation. These results suggest that latewood is a seasonal recorder of Wg trends, with a temporal lag corresponding to the mixing time of sugars in the phloem. These six trees also showed significant negative correlations between photosynthetic discrimination D calculated from intra-ring d13C, and ecosystem Wt, during latewood formation. Despite the observed between-tree variability, these results indicate that intra-ring d13C can be used to access seasonal variations in past Wt.