Abstract: Model arkoses containing K-feldspar (or Na-feldspar) + kaolinite + quartz (or silica glass or boehmite) were reacted in solutions of a variety of compositions in the KCl-NaCl-H 2 O system, and in sea water at 200 to 350°C and 500–1,000 bars in rapid-quench, cold-seal pressure vessels. Experiments reveal that: (1) mixed-layer illite/smectite (I/S) with or without discrete illite are the major neoformed phases for all runs with low fluid/rock ratio; (2) the precipitation rate for neoformed clays is similar for near-neutral solutions of varying compositions, but the expandability of the I/S depends strongly on solution composition at the same temperature and pressure; (3) discrete illite appears only in solutions rich in K*; (4) sea water dramatically retards the formation of illite layers in I/S; (5) illitic clay with fibrous habit grows more effectively in solutions with silica oversaturated with respect to quartz; illitic clays formed in initially alkaline and acidic solutions tend to be platy; and (6) illitic clays form in near-neutral solutions at much slower rates than those in alkaline solutions. The experimental study verifies that the nonequilibrium mineral assemblage feldspar + kaolinite + quartz serves as a control for fluid composition, which favors precipitation of illitic clays in rock-dominant systems. The diagenetic products of a feldspar-bearing sandstone are determined primarily by the presence or absence of kaolinite and by the flow rate, and secondly by the initial fluid composition or temperature. The initial fluid composition becomes crucial only in the fluid-dominant system, whereas temperature is more important in controlling the kinetics than in shifting the thermodynamics of illitic-clay formation. The commonly observed illitization of kaolinite-bearing arkosic sandstones during burial diagenesis is attributed more to the reduction of the flow rate of the existing fluids than to the influx of new illitization fluids, or to the increase of temperature alone.