Studies of the paragenesis of authigenic illite in arkosic sandstonesof various regions and ages have revealed that the illitizationof kaolinite is an important reaction accounting for the formationof authigenic illite in sandstones during burial diagenesis.The illitization of kaolinite takes place at an intermediateburial depth of 3–4 km, where pressure can reach valuesof 100 MPa (1000 bars). The purpose of the present study wasto analyze the effect of pressure on the rate of kaolinite illitizationin alkaline conditions. Hydrothermal reactions were conductedon KGa-1b kaolinite in KOH solution at 300°C and under pressuresof 500, 1000, and 3000 bars for 1 to 24 h. The visual examinationof the X-ray diffraction (XRD) patterns indicated a notableinfluence of pressure on the reaction rate. Molar percentagesof muscovite/illite formed at each time interval were calculatedfrom the analysis of two diagnostic XRD peaks, representingthe 060 reflections of kaolinite and muscovite/illite. The datawere modeled to obtain the initial rate of conversion at eachpressure. The results indicated that the initial rate of kaoliniteto muscovite/illite conversion is one order of magnitude greaterat 3000 bars than at 500 or 1000 bars. Comparison of these datawith those in the literature show a faster conversion rate (severalorders of magnitude) in an initially high-alkaline solutionthan in a near-neutral solution.