Chloride and sulphate concentrations in rainwater and water-soluble leachates from volcanic ash samples track the compositions of gas emissions at the Soufrière Hills Volcano, Montserrat, from 1996 to 2001. There are both systematic spatial and temporal variations in the chloride/sulphate ratio (expressed as the equivalent HCI/SO2 mass ratio) in rainwater and ash leachates. Temporal variations reflect changes in eruption rate and eruptive style. Mass ratios of HCI/SO2 in ash leachates correspond closely with those obtained by open-path Fourier transform infrared (OP-FTIR) spectroscopy, and reflect changes in volatile emissions throughout the eruption. Both leachate and OP-FTIR spectroscopic analyses show mass ratios of HCI/SO2>1 during dome growth, and HCI/SO2
