Building on the discovery of excess 17O in atmospheric sulfate by Lee et al. (2001), we have carried out a case study to determine whether 17O might provide a new tool for quantifying the impact of atmospheric deposition on surface-water sulfate loads. In Rocky Mountain alpine regions, excess 17O was found to be characteristic of atmospheric sulfate deposited in snow. Excess 17O was also evident in stream sulfate in one of two high-elevation watersheds where analyses were made. Isotope mass balance calculations gave surprizingly low atmospheric contributions to stream sulfate suggesting that (1) despite abundant outcrop and sparse soil in these areas, significant sulfate may be taken up and released by soil microbes before being exported in streams, and (2) surface waters can carry multiple non-atmospheric sulfate types, some possibly anthropogenic. Measurements of 17O may prove very useful in studies of sulfate behavior in a variety of surficial environments. ¿ 2001 American Geophysical Union