Volcanic gases provide important insights into deep-Earth processes, and gas composition and flux variations show promise as predictors of eruptive activity(1-3). But data correlating gas composition with eruptions are sparse, largely because such studies have traditionally involved direct sampling inside a volcanic crater-a hazardous operation that has resulted in numerous deaths(4,5). Crater-rim-based spectroscopy(6-9), closed-path spectroscopy of gases sampled from aircraft(10), and time-averaged studies using volatile traps(11-13) allow measurements to be taken from safer distances. But when a full-scale explosive eruption threatens, even these methods become dangerous as the hazard radius expands to many kilometres. Previously, only sulphur dioxide has been reliably measurable at such large distances, using correlation spectroscopy(14). Here we describe techniques that extend the useful range of passive infrared spectroscopy to monitor many gases at distances of over 17 km. We demonstrate the use of these techniques in a high-temporal-resolution study of short-term compositional variations associated with an explosive eruption at Mexico＇s Popocatepetl volcano on 25-26 February 1997. We observed a steady increase in SiF4/SO2 over several days preceding the eruption, followed by a tenfold decrease in this ratio over a few hours immediately afterwards.