The stable isotopic compositions of snow and meltwater are a very useful tool to investigate water provenances and to increase the accuracy of paleoclimate studies in glacial watershed systems. To better understand the factors that affect the isotopic compositions of snow and meltwater in the western Antarctic Peninsula, the isotopic compositions of snow/ice and meltwater from the Barton Peninsula in Antarctica were examined. The isotopic compositions of snow are more enriched than those of meltwater and the variability of the isotopic compositions decreases from snow to meltwater. The melting process changes the linear slope between two water isotopes, which is different from the meteoric water line. We observe that the isotopic compositions of snow are altered by tephra, which results in isotopically enriched snow that is covered by tephra. This tephra decreases the snow's albedo, and the increased energy that is absorbed by the snow surface increases the melting rate, which causes isotopic exchange between liquid water and ice. Hydrological processes, such as daily variations in the melting rate and contributions from groundwater/runoff to seawater, also affect the isotopic compositions of water over the Barton Peninsula. Our works imply that uncertainty caused by these variations should be considered when applying stable water isotopes in this area for water flowpath and paleoclimate study.