The study of isotopic variation in snowmelt from seasonal snowpacks is useful for understanding snowmelt processes and is important for accurate hydrograph separation of spring runoff. However, the complex and variable nature of processes within a snowpack has precluded a quantitative link between the isotopic composition of the original snow and its melt. This work studies the isotopic composition of new snow and its modification by snow metamorphism and melting. To distinguish individual snowstorms, we applied solutions of rare earth elements to the snow surface between storms. The snowmelt was isotopically less variable than the snowpack, which in turn was less variable than the new snow, reflecting isotopic redistribution during metamorphism and melting. The snowmelt had low Δ18O values early in the season and became progressively enriched in 18O as the pack continued to melt. On a given day, meltwater Δ18O was systematically lower whenever melt rates were low than when melt rates were high. The progressive enrichment in Δ18O of snowmelt and the dependence of Δ18O on melt rates can be explained by isotopic exchange between liquid water and ice. A one-dimensional (1-D) model of the melting process, including advection and water-ice isotopic exchange kinetics, reproduces the observed progressive 18O enrichment of snowmelt. ¿ 2001 American Geophysical Union