During February 1978 a group of scientists from the National Center for Atmospheric Research, several colleges and universities, the U.S. Geological Survey, and NASA used a specially equipped Beech Queen Air aircraft to make 11 sampling flights in Guatemala through the eruption clouds from the volcanoes Pacaya, Fuego, and Santiguito. Measurements were made SO42-, SO2, HCl, HF, and 11 cations that were in water-soluble form, on samples collected by a specially designed filter pack. Particle size distributions were obtained with a piezoelectric cascade impactor, and the particles were identified by energy dispersive X ray analysis. Evacuated canisters were flown to obtain samples for gas chromatographic analysis. Some of the conclusions reached are that since most of the sulfur was found to be in the form of SO2, the H2SO4 droplets resulting from major explosive eruptions must largely result from the reaction of SO2 with OH, at the same time depleting the atmosphere of OH; the volume concentration ratio 2>/ always somewhat exceeded unity; and the amount of fine ash remaining in the stratosphere for long periods of time may depend on the crystallinity of the magma. Correlation spectrometry showed that each volcano was emitting 300--1500 metric tons of SO2 per day.