Volcanic gases carry valuable information about processes occurring at active volcanoes, and so their accurate measurement and analysis are highly desirable. Masaya is a low-lying persistently active basaltic volcano, which is renowned for strong passive gas emission unaccompanied by lava extrusion, although it has also undergone plinian eruptions in the past. Its consistent behaviour sustains a reliable tropospheric plume, making it an ideal location at which to study volcanic degassing. During February-March 1998 and March 1999, Masaya's gas plume composition was investigated using the new ground-based remote sensing technique of open-path Fourier transform infrared spectroscopy. This technique meets criteria for safe and accurate quantitative characterisation of volcanic gases and on a temporal resolution previously impracticable. Flexibility of operation is the chief merit of OP-FTIR, since a range of infrared sources are available (e.g. an active lamp, the sun or hot volcanic vents). Laboratory calibration experiments using primary gas standards confirmed that the instrument and subsequent spectral analysis provide highly accurate concentration measurements for volcanic species, with errors typically around 5 %. Results showed that Masaya's plume composition was little changed between the field seasons. Average molar ratios for S0₂/HCl, HCl/HF, C0₂/S0₂ and H₂0/S0₂ were 1.6, 4.9, 2.2 and 72, respectively, in 1998, and 1.6, 5.1, 2.3 and 66 in 1999. These ratios, coupled with simultaneous COSPEC-derived SO₂ data, indicated that emission rates, especially of HCl and HF, were high compared with other passively-degassing volcanoes. Measurements using the spectrometer with different infrared sources and at different locations downwind demonstrated that tropospheric scavenging processes had little discernible effect on plume composition. in the light of these new OP-FTIR gas data, possible physical mechanisms for the degassing behaviour at Masaya Volcano have been discussed. In order to account for all the observations, an integrated model, in which gas emission rates are primarily controlled by degassing-driven convection in the conduit, is proposed.