Volatile exsolution is widely recognized as an important trigger for eruptions from shallow magma reservoirs, but relatively few studies quantify the effects of exsolution on internal pressure within deeper-seated intrusive bodies. We present a model to predict internal pressure changes during the crystallization of a haplogranite melt containing 3 and 5 mass % H2O and with an emplacement pressure of 200 MPa. Mass and volume relations between phases are used to determine internal pressure assuming a closed, constant-volume system. The results indicate that initial crystallization of alkali feldspar and quartz causes a decrease in pressure prior to the exsolution of an aqueous fluid from the residual melt (i.e., resurgent boiling). Further crystallization toward the core of the body in the presence of a separate volatile phase results in a sharp increase in internal pressure. Our model shows that in closed, isochoric systems, the crystallization of the H2O-saturated melt will generate internal pressures that greatly exceed emplacement pressures typical of miarolitic pegmatites. Extreme overpressure modifies the physical and chemical properties of the residual melt and coexisting aqueous fluid, which in turn influences crystallization kinetics and the development of primary textures. Primary melt and fluid inclusions in pocket minerals thus likely represent samples trapped at various pressures in a rapidly evolving melt–fluid system. In most pegmatites, increasing fluid pressure and the formation of large pockets is regulated by the permeability and tensile strength of the enclosing rock. This explains why many miarolitic pegmatites occur within rigid host rocks such as granite, gabbro, and gneiss.