A rapid heating (980¿C)-gas chromatographic-mass spectrometric technique was developed to measure the SO2 produced from Mt. St. Helens' ash collected after the May 18, 1980 eruption. The average values of evolved SO2 for ash samples from Moses Lake, Missoula, and Helena are 215, 800, and 1250 ppm, respectively. The results suggests that the SO2 is associated primarily with new magmatic material. Experiments indicate that the SO2 is not due to sulfate species scavenged from the eruption plume or to sulfur gases adsorbed on the ash. Other possible sources include reduction of sulfate salts within the ash, bubbles of SO2 trapped within the ash, or sulfur blebs contained in the ash. Approximately as much SO2 or SO2 precursors are associated with the ash as Mount St. Helens injected into the stratosphere.