The oxygen-isotope content of terrestrial plant cellulose is related to that of the source waters by a two-step process involving evapotranspirative leaf-water enrichment and equilibrium isotopic exchange between leaf water and atmospheric carbon dioxide. A combination of two models that describe these steps yields consistent agreement between measured and predicted climatic and isotopic data. Humidity is the dominant influence on variations in the cellulose 18O enrichment relative to the source water. A good first-order approximation of the average daytime relative humidity during the growth season at a site can be based on the linear correlation that exists between humidity and cellulose enrichment, without explicit consideration of factors such as temperature, δ18O of atmospheric vapour, and leaf boundary-layer dynamics.The value of the combined model for paleoclimatic reconstruction has been tested using fossil wood from a late glacial site at Brampton, Ontario. Estimates of the past relative humidity were derived from the divergence between measured δ18O values of fossil wood cellulose and environmental water isotopic compositions inferred from the carbon-bound deuterium contents of the cellulose. Growing conditions were apparently substantially drier than those at present between about 11 500 and 8700 years BP, at a time when coniferous forests predominated in southwestern Ontario. A shift in the inferred meteoric water composition over this period suggests a gradual increase in mean annual temperature of about 2 or 3 °C, in agreement with estimates of temperature change based on paleoentomological data.