We measured the carbon and oxygen isotopic composition of stem cellulose of Piiius sylvestris, Picea abies, Fagiis sylvatica and Fraxinus excelsior. Several sites along a transect of a small valley in Switzerland were selected whieh differ in soil moisture conditions. At every site, six trees per species were sampled, and a sample representing a mean value for the period from 1940 to 1990 was analysed. For all species, the mean site ^''C and ()'**O of stem cellulose are related to the soil moisture availability, wherehy higher isotope ratios are found at drier sites. This result is consistent with isotope fractionation models when assuming enhanced stomatal resistance (thus higher ^'''C of incorporated earhon) and increased oxygen isotope enrichment in the leaf water (thus higher ^'^'O) at the dry sites. 5'*'O-5'"'C plots reveal a linear relationship between the earhon and oxygen isotopes in cellulose. To interpret this relationship we developed an equation whieh combines the above-mentioned fraetionation models. An important new parameter is the degree to whieh the leaf water enrichment is reflected in the stem cellulose. In the combined model the slope of the 5"^O-5"C plot is related to the sensitivity of the z?//?,, of a plant to changing relative humidity.