The chemical composition of volcanic gases from the summit crater of Sakurajima Volcano was observed by a variety of gas collection techniques and analytical methods, and the H2 content in gas from a hot spring was continuously measured by an automatic gas chromatograph. Increases in the molar ratio of HCl to SO2 were found to coincide with, and sometimes to precede, rises in volcanic activity. An increase in H2 content of the hot spring gas was also found to precede a period of increased volcanic activity.