New analytical and experimental data constrain the storage and equilibration conditions of the magmas erupted in 1912 from Novarupta in the 20th century's largest volcanic event. Phase relations at H₂O+CO₂ fluid saturation were determined for an andesite (58.7 wt% SiO₂) and a dacite (67.7 wt%) from the compositional extremes of intermediate magmas erupted. The phase assemblages, matrix melt composition and modes of natural andesite were reproduced experimentally under H₂O-saturated conditions (i.e., P_H2O=P_TOT) in a negatively sloping region in T-P space from 930 °C/100 MPa to 960 °C/75 MPa with fO₂~N NO + 1. The H₂O-saturated equilibration conditions of the dacite are constrained to a T-P region from 850 °C/ 50 MPa to 880 °C/25 MPa. If H₂O-saturated, these magmas equilibrated at (and above) the level where coerupted rhyolite equilibrated (~100 MPa), suggesting that the andesite-dacite magma reservoir was displaced laterally rather than vertically from the rhyolite magma body. Natural mineral and melt compositions of intermediate magmas were also reproduced experimentally under saturation conditions with a mixed (H₂O + CO₂) fluid for the same range in P_H2O. Thus, a storage model in which vertically stratified mafic to silicic intermediate magmas underlay H₂O-saturated rhyolite is consistent with experimental findings only if the intermediates have X_H2Ofl=0.7 and 0.9 for the extreme compositions, respectively. Disequilibrium features in natural pumice and scoria include pristine minerals existing outside their stability fields, and compositional zoning of titanomagnetite in contact with ilmenite. Variable rates of chemical equilibration which would eliminate these features constrain the apparent thermal excursion and re-distribution of minerals to the time scale of days.