The Mount St. Helens, May 18 pumice is a dacite containing 60% glass by weight and phenocrysts of plagioclase, orthopyroxene, amphibole, titaniferous magnetite, and ilmenite. The glass is uniform in composition, a rhyodacite with 73 wt % SiO2; the phenocrysts are also uniform in composition except for the plagioclase, which has cores averaging An57 and rims averaging An49. Analyses of seven pairs of coexisting Fe-Ti oxides in a representative sample of the light pumice were recast using various mineral calculation procedures; they yielded temperatures ranging from 920¿ to 940¿C and a-log f02 of 10.3--10.1. Electron microprobe analyses of 57 glass inclusions trapped in plagioclase phenocrysts in the light pumice showed little deviation from an average rhyodacitic composition (69.90¿0.87 wt % SiO2) when special care was taken to account for Na loss during the analysis. The difference between the average total of these glass inclusion analyses and 100% is 4.6¿1 wt %, which is interpreted to be volatiles dissolved in the glass. On an anhydrous basis the average glass inclusion composition is identical to the matrix glass, indicating that neither underwent significant fractionation after melt was trapped by the plagioclase. Experimentally determined phase relations for the representative dacite sample place limits on conditions in the May 18 Mount St. Helens magma chamber, assuming that the dissolved volatiles were 4.6¿1 wt % and the temperature was 920¿-940¿C. Hydrothermal experiments over a range of P, T, and f02 indicate that at no pressure is the observed phase assemblage and residual melt chemistry produced when PH20=PTotal. Experiments using CO2-H2O fluids to achieve PH20's less than PFluid did reproduce the observed residual melt chemistry and an An50 plagioclase at a specific set of conditions, i.e., at f02's between the NNO and MNO buffers, at a PFluid of 220 MPa (2.2 kb), and at a PH2O=110 MPa (all at 920¿-940¿C). Amphibole was not stable under those conditions but possibly would be if the PH20/PFluid ratio was raised to 0.7 or if fluorine were added to the experimental system. It is concluded that just prior to eruption, the upperpart of the Mount St. Helens magma chamber was at a pressure of 220¿30 MPa corresponding to a depth of 7.2¿1 km, PH20 was 0.5 to 0.7 PTotal, and the temperature was 930¿¿10¿C.