The 1992 and 1995 basaltic eruptions oi Cerro Negro volcano, Nicaragua, had contrasting eruptive styles. Although they were nearly identical in composition, the 1992 eruption was explosive, producing a 7 kilometer-high sustained ash column, whereas the 1995 eruption was essentially effusive. The differences in water and carbon dioxide contents of melt inclusions from the two eruptions define minimum saturation pressures and show how decompression of initially similar magmas influences eruptive style. Before eruption, the explosive 1992 magma retained water and carbon dioxide while ascending to a moderate crustal level (about 6 kilometers), whereas the nonexplosive 1995 magma lost all carbon dioxide by degassing during ascent to shallow crustal levels (about 1 to 2 kilometers).