An experimental kinetic study was performed to assess the effect of pressure on vitrinite reflectance (VI?). A series of confined system maturation experiments was conducted on xylite of swamp cypress at 450 degrees C, pressures of 10 and 20 kbar and for run lengths from Os to 25 days. A strong experimental effect of heat-up to 450 degrees C on VR is evident and must be corrected in kinetic analysis. This heat-up effect demonstrates the initial vitrinite maturation kinetics to be very rapid. At 450 degrees C, VR significantly increases with increasing heating time at each investigated pressure. The increase in VR with time (t) at 450 degrees C and pressure (P) from an initial VR of 0% is accurately described by the power law rate equation VR(P, 450 degrees C, t) = (k(P, 450 degrees C)t)(n(P, 450 degrees C)) where the exponent n(P, 450 degrees C) and the rate constant k(P, 450 degrees C) increase with pressure. We consider this kinetic equation a further step toward a general formulation describing VR evolution as a function of time, pressure and temperature for Type III organic matter. The effect of pressure on VR at 450 degrees C is similar to that at 400 degrees C. Obviously, each magnitude of VR has its own increase kinetics, which is pressure and temperature dependent. VR increase kinetics diminishes with increasing VR, resulting in VR increase deceleration with increasing time after rapid initial kinetics. Whereas the promoting influence of temperature on VR is evident, the role of pressure on VR increase kinetics and therefore on Type III organic matter maturation at 400-450 degrees C is complex: It depends on the magnitude of VR with respect to the key VRi = 134 +/- 029%, at which a transition in the effect of pressure on vitrinite maturation takes place. For VRVRi, pressure promotes vitrinite maturation inasmuch as increasing pressure reduces VR increase deceleration with increasing VR and thus greatly augments VR increase with increasing time. This promoting effect of pressure increases with increasing VR. Therefore, at 400-450 degrees C, the overall kinetics of the physico-chemical processes occurring during peatification, dehydration and bituminization are pressure retarded, those taking place roughly during the first half of debituminization are almost pressure independent and those proceeding during graphitization and the second half of debituminization are pressure favored. At 400-450 degrees C, pressure promotes vitrinite maturation in geological settings because the key Vie is attained in only a few minutes to a few hours and therefore the retardation is insignificant. (C) 2012 Elsevier B.V All rights reserved.