We explore the potential of tree-ring cellulose delta O-18 and delta C-13 records for reconstructing climate variability in the southeast Tibetan Plateau. Our sampling strategy was designed to investigate intra and inter-tree variability, and the effects of the age of tree on delta O-18 variation. We show that intra-tree delta C-13 and delta O-18 variability is negligible, and inter-tree coherence is sufficient to build robust tree-ring delta O-18 or delta C-13 chronologies based on only four trees. There is no evidence of an age effect regarding delta O-18, in contrast with tree-ring width. In our warm and moist sampling site, young tree delta C-13 is not clearly correlated with monthly mean meteorological data. Tree-ring delta O-18 appears significantly anti-correlated with summer precipitation amount, regional cloud cover, and relative humidity. Simulations conducted with the ORCHIDEE land surface model confirm the observed contribution of relative humidity to tree cellulose delta O-18, and explain the weak correlation of delta C-13 with climate by the non-linear integration linked with photosynthesis. Altogether, the tree-ring cellulose delta O-18 is shown to be a promising proxy to reconstruct regional summer moisture variability prior to the instrumental period. (C) 2010 Elsevier B.V. All rights reserved.