The stable isotope oxygen!07 is often used as a natural tracer in stream!~ow separation studies in forested catchments[ Knowledge about the 07 O content of throughfall and soil water is needed[ The present study was conducted in order to assess the e}ect of vegetation structure on the stable isotope 07 O composition of rainwater input during its passage through the canopy and into the unsaturated zone[ The research area was a small "9=0 km 1 # forested catchment in the mountainous {Black Forest| region of southern Germany[ During the 2!month periods "September to December 0884 and April to July 0885# several structural units were instrumented according to tree species "beech and spruce# and canopy density[ Overall throughfall was enriched compared with open rainfall by Β¦9=27-and Β¦9=25-for spruce and beech\ respectively[ Considerable di}erences existed in the results of the crown centre and the crown periphery sites[ Throughfall in the crown centre showed a higher degree of enrichment\ with a mean value of Β¦9=29for beech and Β¦9=26-for spruce[ Enrichment and depletion were observed in the lower canopy density locations[ Mean results of the crown periphery were similar to those of open precipitation "Β¦9=04-for beech and -9=90-for spruce#[ Soil water was sampled at _ve di}erent depths "9\ 04\ 59\ 019 and 079 cm#[ The signals of individual rainfall events could be traced down to a depth of 59 cm[ The soil water in the upper layers followed the seasonal trend in the precipitation input[ At a depth of 079 cm\ soil water had a very constant d 07 O value[ The 07 O composition of the soil water at various depths at di}erent locations showed a similar behaviour[ No detectable di}erences could be found between the structural units in the di}erent layers\ except at 079 cm depth[ This might be attributed to downslope directed ~ow at that depth [ Copyright Γ 1999