AbstractThe differences between δ18O in throughfall and open rainfall were studied for 16 selected spring and summer storms in deciduous, pine and spruce forests in central Pennsylvania, USA. Throughfall δ18O averaged 0.17, 0.32 and 0.24%ogreater than δ18O of open rainfall for all storms at the deciduous, pine and spruce sites, respectively. Throughfall18O enrichment was greater in low intensity spring rainfall events than higher intensity growing season storms and greater in the coniferous stands than the deciduous hardwood stand. Maximum throughfall18O enrichment of l.61%ooccurred in the spruce forest during one spring event. The differences between rainfall and throughfall18O observed in this study for individual storm events may have important implications for isotope flow separation studi