To clarify the precipitation behavior of silica from a hydrothermal solution flowing with decreasing temperature through rock, the effects of temperature, temperature gradient, pH, flow velocity, and solid surface area/fluid mass ratio (A/M ratio) were analyzed by experimentation. Although these effects were not evaluated precisely, we found ''threshold conditions'' where the solution remains in equilibrium with amorphous silica in a flow field with a negative temperature gradient. The equilibrium conditions are: the mean pore velocity is less than 100 m yr-1, when A/M ratio is more than 700 m2 kg-1, temperature is 80¿-120 ¿C, temperature gradient is <50 ¿C m-1, and pH is 6.5 to 8. For the same A/M ratio, temperature gradient, and pH, the mean pore velocity must be less than 5 m yr-1 to keep the solution in equilibrium with the amorphous silica in a flow field at 80¿--25 ¿C. These threshold conditions are probably satisfied in many natural geothermal systems and near a repository for high-level radioactive waste (HLW), suggesting that once groundwater becomes saturated with amorphous silica along the flow path from a geothermal source or HLW repository, it would flow in equilibrium with amorphous silica. ¿ American Geophysical Union 1994