A circulating apparatus has been specifically designed for kinetic studies of mineral-water interactions in hydrothermal systems. The equipment can be operated to 350°C and 140 bars with flow rates of 0.5 to 320 mL/hr. It can be used in a variety of modes depending on the application: semi-static, continuously-fed stirred tank reactor (CSTR), plug flow and packed-bed. The theory of operation and derivation ofkinetic rate laws in each basic mode is outlined. Experimental examples are presented for the dissolution kinetics of calcite, fluorite and magnetite in simple aqueous systems to 350°C and 140 bars using CSTR, packed-bed plug flow and semi-static modes.