Measurements of δ34S covering the years 1935–76 and including the 1963 Agung (Indonesia) eruption were made on a West Antarctic firn core, RIDSA (78.73˚ S, 116.33˚ W; 1740ma.s.l.), and results are used to unravel potential source functions in the sulfur cycle over West Antarctica. The δ34S values of SO42– range from 3.1‰ to 9.9‰. These values are lower than those reported for central Antarctica, from near South Pole station, of 9.3–18.1‰ (Patris and others, 2000). While the Agung period is isotopically distinct at South Pole, it is not in the RIDSA dataset, suggesting differences in the source associations for the sulfur cycle between these two regions. Given the relatively large input of marine aerosols at RIDSA (determined from Na+ data and the seasonal SO42– cycle), there is likely a large marine biogenic SO42– influence. The δ34S values indicate, however, that this marine biogenic SO42–, with a well-established δ34S of 18‰, is mixing with SO42– that has extremely negative δ34S values to produce the measured isotope values in the RIDSA core. We suggest that the transport and deposition of stratospheric SO42– in West Antarctica, combined with local volcanic input, accounts for the observed variance in δ34S values.