In this study the temperature profiles inside a tube batch reactor under the sub-critical water condition were investigated under different operating conditions. The most important parameters that may affect the temperature profile, including salt bath mixing speed, reactor shaking, reactant viscosity, and reactor contents, were investigated both experimentally and theoretically. The results showed that both salt bath mixing and reactor shaking had a great effect in the rate of heating up. The study of the effect of reactant viscosity on the temperature profile revealed that the thermal conductivity rather than viscosity was found to affect the temperature profile in the case of comparing reactant with different viscosities. The reactant contents had the lowest pronounced effect on both heating up and cooling down rates among all tested parameters. Two different empirical equations were developed to describe the temperature profiles during both heating up and cooling down periods. Using these equations we developed a new mathematical model for describing the kinetic behavior of fast reactions that take place under the sub-critical water condition. We assumed that the reaction was taking place under unsteady state conditions and following first order kinetics. Based on the simulation results, we developed a new strategy for finding out the correct values of activation energy and frequency factor involved in such fast reactions. (c) 2006 American Institute of Chemical Engineers.