AbstractInformation about the transition state of a chemical reaction is quite useful for the determination of mechanisms and for design of solvents for the reaction. High pressure kinetic studies, if sufficiently accurate, provide the required data. This work reports a new technique developed for measuring rates of chemical reactions in solution at high pressure. This method involves in situ mixing of reactants under pressure and direct sampling for analysis, eliminating large errors of temperature equilibration and time measurement inherent in existing techniques, and thus yielding more accurate results. Experimental results for the Diels‐Alder reaction of isoprene and maleic anhydride are reported at pressures up to more than 6,000 atm.