The oxygen isotope content of nocturnal ecosystem respiration (δ18OR) was examined in forests along a precipitation gradient in Oregon, USA, to determine whether site-to-site variation in δ18OR was more strongly related to variation in δ18O of precipitation or to evaporative processes that isotopically modify water pools within ecosystems. Measurements were made over 4 years at sites ranging in mean annual precipitation from 227 to 2760 mm. There was a gradient in the isotopic content (δ18O) of precipitation, with inland sites receiving isotopically depleted precipitation (more negative δ18O) relative to coastal sites. The δ18O of water in plant xylem generally followed the isotopic pattern of precipitation. Inland forests were drier than coastal forests, leading to a gradient in the vapor pressure deficit of air that caused isotopic enrichment of soil and leaf water. The enriched soil and leaf water pools influenced the isotopic composition of respired CO2, leading to variation in observed δ18OR (Keeling-plot intercepts). Keeling plots with non-significant (p > 0.01) regression slopes and those sampled over a time period (t) greater than 5 hours yielded unacceptably high uncertainty in δ18OR. The range of observed δ18OR was 21.7 to 35.3‰ (VSMOW), with variation within a single site as large as 10.7‰ (range 24.2 to 34.9‰ at different sites). The results suggested a trend of more positive δ18OR at inland sites relative to those nearer the coast, indicating that fractionation due to evaporative enrichment overshadowed the original isotopic composition of precipitation as a first order control on δ18OR.