The evolution of an aqueous vapor phase from a simplified quartz monzonite magma during intrusion and crystallization has been quantitatively modeled using the thermal distribution for a stock of square cross section, calculated according to the simplified theory of Jaeger (1969) and modified by an initial temperature distribution.During the intrusion of a body, the portion intruded at pressures significantly less than that of a certain intersection point (P 1 ) on an isocompositional P-T diagram will generate a vapor phase. The pressure of the point P 1 is dependent on the water content and bulk composition of the magma, while the exact pressure at which a vapor is generated is also dependent on temperature. Upon crystallization, those parts of the body at pressures less than P 1 will actively generate a vapor phase, forming a vapor-saturated cap. Those portions at pressures greater than that of another intersection point P 2 will generate a vapor phase only within a narrow region adjoining the solidifying front of the magma where crystallization has proceeded sufficiently to reach vapor saturation. Any mass movement can cause the entire magma column undergoing the process to be leached of part of its volatile content if the magma passes through the zone of vapor saturation. The model may be directly applied to the generation of magmatic fluids in stocks associated with porphyry copper deposits. The generation of such an aqueous vapor, in conjunction with connate and meteoric waters circulating through the crystalline portions of the body and surrounding rocks, may be important in the generation of hydrothermal solutions which are responsible for the deposition of porphyry copper deposits.