The Columbia River Basalt Group (western United States) is Earth’s youngest and best-studied flood basalt province, but attempts to link it with mid-Miocene global change have proven elusive. Part of the difficulty lies in a lack of comprehensive data for volatile emissions from the most intense main phase of activity. We present measurements of sulfur contents in magmas that erupted to form the Wapshilla Ridge Member and associated units of the Grande Ronde Basalt, the most voluminous portion of the Columbia River Basalt Group, erupted at the time of peak magma flux. We sampled melt inclusions and host glasses preserved in near-vent phreatomagmatic deposits associated with the voluminous lavas. Sulfur contents of melt inclusions range up to 0.19 wt% S, while host glasses are variably degassed with 0.01–0.13 wt% S. Incomplete degassing of glassy lapilli is attributed to phreatomagmatic quenching in the vent. The magmatic S contents in the very voluminous (∼40,000 km3) Wapshilla Ridge Member scale up to 242–305 Gt SO2 release to the atmosphere over a maximum time period of 94 k.y. The time of the eruption is close to that of a global temperature drop near the peak of the Miocene Climatic Optimum, but refinement of eruption tempo is needed before a cause-and-effect relation can be established.