Over 50 whole-air gas samples were collected in Mt. St. Helens volcanic plumes beginning with the original steam and ash plumes in March 1980 and continuing through June 1980. Four research aircraft were used to collect the samples, which were analyzed by five different analytical techniques for 14 separate trace gases. COS, CS2, CH3Cl, CO2, CO, N2O, C2H6 and C2H2 were found in elevated amounts in the volcanic plume gases. Order-of-magnitude estimates of COS, CS2, CH3Cl, CO2 and CO fluxes have been made for several types of eruptive phases. COS was a substantial fraction of the sulfur-carrying gases during the May 18 eruptions, although SO2 and H2S still predominated.