We established the phase equilibria of a representative tephriphonolitic sample of the 1631 eruption of Vesuvius (Italy). Experiments were conducted at 100 MPa, in the temperature range 950-1050 °C for melt water content ranging from 1.3 to 3.2 wt%, and at an oxygen fugacity (fO2) of NNO+1 to NNO+3 (one to three log unit above the fO2 of the Ni-NiO solid redox buffer). Results show that clinopyroxene, biotite and leucite dominate the crys-tallizing phase assemblage, with minor proportions of plagioclase and amphibole, in agreement with the petro-logical attributes of the tephra. Comparison between the phase proportions and compositions obtained in experiments and those observed in the rock indicates a pre-eruptive temperature of 950 ± 30° and a melt water content H2Omelt = 2.3 ± 0.3 wt%, for an oxygen fugacity around NNO+1. These T-H2O estimates are con-firmed by empirical geothermometers based on experimental clinopyroxene and melt compositional trends. As for other Vesuvius eruptions, the most felsic part of the 1631 reservoir appears to have reached pre-eruptive leucite saturation, although a large amount of leucite microcrystals in the studied samples likely grew syn-eruptively. Our results confirm that magma storage conditions beneath Vesuvius became hotter, shallower, and more CO2-rich after the AD 79 Pompeii Plinian event.