Volcanic unrest at Mount Pinatubo began on April 2 with a series of small steam-driven explosions from vents high on the volcano and culminated 10 weeks later in the world's largest eruption in more than half a century. During this pre-eruption period, scientists of the Philippine Institute of Volcanology and Seismology and U.S. Geological Survey monitored the sulfur dioxide gas emissions using a correlation spectrometer (COSPEC) mounted in a helicopter and a small airplane.

Measurements of the sulfur dioxide emission rate from the volcano began on May 13. The measurements showed an increase in sulfur dioxide gas emission from about 500 tonnes per day to more than 5,000 tonnes per day two weeks later. The tenfold increase implied that magma was rising toward the surface and (or) that the volcano's hydrothermal system that absorbed volcanic gases was being boiled and thus removed, allowing more sulfur dioxide gas to escape into the atmosphere.

A sudden, short-lived drop in sulfur dioxide output to about 260 tonnes per day in early June may have been caused by plugging or sealing of magma and fractures through which gas was escaping. Then the emission rate increased again as lava erupted on the surface to form a dome. The last measurement before the large explosive eruptions began on June 12 was more than 13,000 tonnes per day!