We report non-eruptive plume gas composition data collected by open-path Fourier transform infrared (OP-FTIR) spectroscopy at the summit of Masaya Volcano, Nicaragua, in 1998 and 1999. Masaya's plume is characterized by SO2/HCl and HCl/HF mass ratios of 2.8--3.0 and 8.1--8.2 respectively. When combined with COSPEC SO2 emission rate data, these ratios indicate average emission rates of HCl and HF of 7.3 kg s-1 and 0.9 kg s-1. The gas composition is remarkably consistent, showing little temporal variation on the scale of days, weeks or even between the two observation periods, one year apart. Such striking uniformity is best explained by steady-state open system degassing. ¿ 1999 American Geophysical Union