The effects of fluid-rock interaction at hydrothermal conditions were investigated by a detailed study of altered quartz diorite around a single gold-bearing quartz vein, and by experiments reacting quartz diorite with water, using a single-pass, continuous-flow system. Alteration in the natural environment produced ankerite and albite distal to the vein, and albite adjacent to it. Mass balance calculations show that there was a progressive mass loss (up to 30%) towards the vein, due primarily to the dissolution of quartz. In experiments at 200$ sp circ$C, SiO$ sb2$, Na and CO$ sb2$ concentrations were controlled by the dissolution of quartz, albite and ankerite. SiO$ sb2$ reached a steady state concentration below quartz saturation. The concentrations of Ca, Mg, Fe, and Al were buffered by the precipitation of smectite, boehmite and calcite. These experiments show that a fluid may not reach quartz saturation at 200$ sp circ$C, especially if the infiltrating fluid contains dissolved components which can promote the formation of secondary, quartz-consuming minerals. In the experiment at 350$ sp circ$C, the fluid reached quartz saturation almost immediately. The secondary minerals were anorthite, chlorite, boehmite, and titanite, which formed at the expense of albite, calcite, and ankerite, thereby increasing pH and releasing Na, SiO$ sb2$, CO$ sb2$ and Al to the fluid. Quartz was completely dissolved within 500 hours. The most significant conclusion from this experiment is that the progressive loss of quartz to a quartz-undersaturated fluid increases the surface area of more resistant minerals and promotes their dissolution. Investigations of both the natural and experimental systems emphasize the importance of quartz dissolution in creating porosity, and mineral reaction kinetics in controlling wall-rock alteration.