In response to 12 days of seismic activity at Axial Volcano, Juan de Fuca Ridge in January--February 1998, the NSF/NOAA-sponsored Axial Response Team conducted three response cruises in February, August, and August/September to map and sample hydrothermal plumes over the region. Vertical profiles of particulate Fe and S over the eruption site show high concentrations from about 1400 m to the bottom. Chemical and scanning electron microscope analysis of the February plume samples revealed anhydrite, Fe-ferrihydrites, elemental sulfur, and angular glassy basalt shards up to 190 &mgr;m in the longest dimension. Many of these shards had halite coatings, which is consistent with subseafloor basalt seawater reactions at temperatures >450 ¿C at 1500 m depth. In August and September cruises the basalt shards were no longer present in the hydrothermal plumes. Instead, the plumes were predominantly composed of Fe-ferrihydrites, elemental sulfur, and sulfur filaments. A unique feature of this data set is the high concentrations of elemental sulfur in the lower 60 m of the water column. The sulfur results are suggestive of a significant enrichment of the bacterial biomass in the water column over the eruption site with time. Within the Axial caldera, approximately 10--20% of the total sulfur is present as sulfur filaments. These post-eruption particle compositional changes have strong similarities to the results from the 1993 CoAxial eruption on the Juan de Fuca Ridge. ¿ 1999 American Geophysical Union