Calculations based on existing thermodynamic models for silicate melts are used to model the physical and chemical evolution of the 1984 Mauna Loa magma. Specifically, the calculations simulate the vesiculation-induced crystallization of the ascending magma. The calculations provide a physiochemical model to explain the increase in crystallization under isothermal conditions. The evolution of the Mauna Loa magma is modeled in three separate stages corresponding to (1) the subsurface crystallization of the phenocryst assemblage, (2) the crystallization and vesiculation of the magma during its ascent from depth, and (3) the crystallization and vesiculation of the lavas at the surface. The preeruptive crystallization of olivine and orthopyroxene phenocrysts began at ≂1155¿ C and 0.2 GPa. Olivine and plagioclase alone crystallized during the ascent of the magma. The bulk of the crystallization and vesiculation of the Mauna Loa magma occurred at pressures less than 20 MPa. Under lithostatic pressures this corresponds to depths as shallow as 600--700 m. Ancillary calculations have established the net heat effect of the prescribed ascent path and demonstrate that the heat associated with vesiculation easily compensates for the heats of crystallization. The calculations suggest that the near-surface processes can be quite endothermic depending on the initial H2O content of the magma. For a magma with 1 wt % dissolved H2O the heat balance demands approximately 5--10¿ C cooling. Further calculations predict the liquid line of descent for this ascent path and document the corresponding variations in the physical properties of the melt phase. ¿ American Geophysical Union 1987