We report the first spectroscopic measurements of volcanic gas emissions by lunar occultation. The experiment was carried out at Masaya volcano, Nicaragua in 1998 using a portable Fourier transform infrared spectrometer. Both SO2 and HCl column concentrations were determined to yield a SO2/HCl molar ratio of 2.2±0.28 (±1σ). This is significantly greater than the equivalent ratio of 1.6±0.02 (±1σ) derived from solar occultation measurements of the volcanic plume. We propose that the cause of the nocturnal increase in SO2/HCl ratio is dissolution of HCl into volcanic water droplets within the plume. This arises because the low saturated vapour pressure of water by night results in strong condensation of plume water vapour whereas by day only negligible plume water vapour condenses.