An eruption of Kilauea Volcano, Hawaii, normally results in a decrease in the internal pressure of a summit magma reservoir as it partly drains. The interior of the volcano responses to the decrease in pressure (1) by elastic expansion of remaining magma and gas, (2) additional exsolution of gas dissolved in the magma, and (3) by contraction of the volcanic edifice. This process is modeled here using gravity, geodetic, and observational data for recent events at Kilauea. During magma removal, the ratio of pressure change in the magma reservoir to the volume of associated summit subsidence ranges from less than 0.22 Pa/m3 to more than 0.43 Pa/m3. Lowe ratios occur during large subsidences because of a large effective volume of the reservoir. Small events may involve source volumes of only 2 km3. In contrast, a volume of near 13 km3 is associated with larger events because the boundary between plastic and elastic behavior may migrate outward with increased time and strain. The aggregate compressibility of Kilauea's reservoir is significantly increased by volumetric changes of the CO2 gas phase by bulk compression and solution with pressure. This effect is more pronounced for relatively shallow sources because the bulk modulus of gas is proportional to pressure. The average reservoir CO2 weight fraction may be near 0.0005 to 0.0009. CO2 appears to accumulate in the upper portion of the reservoir where the concentration reaches 0.0030 weight fraction. Gradual long-term subsidences are different from the short-term deflations associated with specific eruptions. During long-term subsidence, no net change in mass or pressure occurs within the summit reservoir. Rifting of the summit area, which may tend to shorten the reservoir as it widens, or slow degassing and contraction of stored magma may cause long-term subsidence. ¿ American Geophysical Union 1992