A telescope-attached FT-IR spectral radiometer was applied for remote measurement of chemical composition of the volcanic gas of Unzen volcano, Japan, which started erupting in November 1990. The observations were carried out at the site about 1.3 km away from the gas-emitting region of the volcano in March and July 1992. Absorption features of SO2 (&ngr;1) and HCl were identified and column amounts of SO2 and HCl of the volcanic gas plume were estimated using a nonlinear least-squares method. The SO2/HCl ratio was in the range between 0.7 and 1.8 for both March and July observations. This is the first remote ground-based observation of HCl in volcanic gas. ¿ American Geophysical Union 1993