Mt. Somma-Vesuvius (Naples, Italy) has erupted potassium-rich and silica-undersaturated products during a complicated history of plinian and non-plinian events. Coarse-grained cognate nodules are commonly found in the pyroclastics and are upper crustal in origin. We examined cumulate and subeffusive nodules from the 3800 y.B.P. Avellino. A.D. 79 Pompei, and A.D. 472 Pollena eruptions. Silicate-melt and liquid-vapor fluid inclusion studies in clinopyroxene from both types of nodules have been used to assess the fluids attending crystallization and to place constraints on the pressure and temperature of nodule formation. Thermometric and volumetric data from primary and pseudosecondary CO2-H2O and CO2 and coeval silicate-melt fluid inclusions indicate that they were trapped at a pressure of ∼1 to ∼2.5 kbar at ∼1200°C. This suggests a crystallization depth of ∼4 to ∼10 km. The H2O-bearing fluid inclusions are abundant from plinian eruptions in contrast to non-plinian eruptions where H2O-bearing fluid inclusions were rare. The presence of primary H2O-CO2 fluid inclusions indicates that an immiscible, supercritical H2O-CO2 fluid was in the nodule-forming environment. The H2O-bearing fluid inclusions in plinian nodules may record a higher pre-eruptive H2O content in the bulk magma that is dramatically reflected in the eruption dynamics.