The carbon and oxygen isotopic compositions of tree ring cellulose were examined for trees along a precipitation gradient in western Oregon, United States. Two years of cellulose from four sites dominated by coniferous forests ranging in precipitation from 227 to 2129 mm were sampled in conjunction with studies that measured the δ18O and δ13C of ecosystem respiration. The mean tree ring cellulose δ13C varied from −22.1 to −26.3‰ among sites and showed enrichment with decreasing water availability across the transect. The δ13C in cellulose varied across the precipitation transect in a similar pattern to the δ13C of leaf and root tissues as well as ecosystem respiration, although tree ring cellulose was enriched in 13C by over 3‰ compared to other organic matter components. The mean tree ring cellulose δ18O varied from 28.1 to 30.3‰. However, trends of cellulose δ18O change with water availability were obscured by differences in stem water δ18O. When calculated as deviation from stem water (δ18Ocellulose − δ18Ostem water) the differences in evaporative enrichment between sites was more pronounced (range of 9.6‰). The limited observed variation in tree ring cellulose δ18O of field grown trees despite large site difference in stem and leaf water δ18O across the transect agreed with predictions from a mechanistic model. Tree ring records of cellulose δ18O may provide useful proxy information regarding humidity and site water balance especially if combined with δ13C records that also vary with plant water status.