Ground-based and aircraft correlation spectrometer (COSPEC) measurements at the principal active volcanoes in Guatemala (Pacaya, Santiaguito, Fuego, and Tacaná) and El Salvador (Santa Ana and San Miguel) were carried out at intervals during the period 1999–2002, as part of an attempt to measure baseline SO2 emissions of potentially dangerous volcanoes and to better understand their eruption mechanisms. We discuss some of the uncertainties involved in interpreting intermittent gas data, together with possible improvements. Other problems pertaining to current monitoring of SO2 and implications for future studies are also discussed. Santa Ana volcano is proposed to be a venting hydrothermal system, while Santiaguito, Fuego, Pacaya, and San Miguel all exhibit open-vent characteristics. Data for Tacaná volcano are presented, but are not enough to make descriptions of its characteristics and activity. Pacaya is emitting high fluxes of SO2 (N1000 tonnes/day), while the other vents are much lower emitters (20–300 tonnes/day in general). SO2 emissions at Pacaya suggest a large circulating and convecting high level chamber. The most recent emission rates at Fuego were measured during its current active phase (since January 2002). Average SO2 emission rates during 1999–2002 are: 1350 tonnes/day for Pacaya, 340 tonnes/day for Fuego, 120 tonnes/day for Santiaguito, 260 tonnes/day for San Miguel, 140 tonnes/day for Santa Ana, and 30 tonnes/day for Tacaná. These volcanoes account for about 6% to 12% of the estimated annual global volcanic output of SO2. D 2004 Elsevier B.V. All rights reserved.