Reheated silicate melt inclusions in volcanic rock samples from Mt. Somma-Vesuvius, Italy, have been analyzed for 29 constituents including H2O, S, Cl, F, B, and P2O5. This composite volcano consists of the older Mt. Somma caldera, formed between 14 and 3.55 ka before present, and the younger Vesuvius cone. The melt inclusion compositions provide important constraints on pre-eruptive magma geochemistry, identify relationships that relate to eruption behavior and magma evolution, and provide extensive evidence for magmatic fluid exsolution well before eruption. The melt inclusion data have been categorized by groups that reflect magma compositions, age, and style of eruptions. The data show distinct differences in composition for eruptive products older than 14.0 ka (pre-caldera rocks) versus eruptive products younger than 3.55 ka. Moreover, pre-caldera eruptions were associated with magmas relatively enriched in SiO2, whereas eruptions younger than 3.55 ka (i.e., the syn- and post-caldera magmas which generated the Somma caldera and the Vesuvius cone) were derived from magmas comparatively enriched in S, Cl, CaO, MgO, P2O5, F, and many lithophile trace elements. Melt inclusion data indicate that eruptive behavior at Vesuvius correlates with pre-eruptive volatile enrichments. Most magmas associated with explosive plinian and subplinian events younger than 3.55 ka contained more H2O, contained significantly more S, and exhibited higher (S/Cl) ratios than syn- and post-caldera magmas which erupted during relatively passive interplinian volcanic phenomena.