A supereruption that occurred in the Campi Flegrei area, Italy, ca. 39 ka had regional- and global-scale environmental impacts and deposited the Campanian Ignimbrite (CI). We attempt to shed light on critical aspects of the eruption (depth of magma chamber, intensive pre-eruptive magma conditions) and the large-volume magma plumbing system on the basis of information derived from analyzing melt inclusion (MI) data. To achieve these aims, we provide new measurements of homogenization temperatures and values of dissolved H2O within phenocryst-hosted MIs from pumices erupted during different phases of the CI eruption. The MI data indicate that a relatively homogeneous overheated trachytic magma resided within a relatively deep magma chamber. Dissolved water contents in MIs indicate that prior to the eruption the magma chamber underwent radical changes related to differential upward movement of magma. Decompression of the rising trachytic magma caused a decrease in water solubility and crystallization, and trachytic bodies were emplaced at very shallow depths. The proposed eruptive model links portions of the main magma chamber and apophyses with specific eruptive units.