The kinetics of deuteration of resorcinol in pure D2O were studied for the first time using a flow-through capillary tubular reactor with on-line, proton, and deuterium NMR detection at high temperatures and high pressure. The global rate constants for hydrogen/deuterium (H/D) exchange were determined from temperatures of 200-450 degrees C (723 K) at a pressure of similar to 400 bar (the critical temperature and pressure of water are 374.2 degrees C and 218.3 bar, respectively). The H/D exchange rate in resorcinol (1,3-dihydroxybenzene) under these extreme conditions was determined using proton NMR as a function of the resorcinol residence time in a capillary tubular reactor, which also served as a high-pressure NMR cell. The H-1 and H-2 NMR results indicate that H/D exchange in resorcinol for the ring protons was observed at temperatures as low as 200 degrees C. The kinetics of H/D exchange in resorcinol and the activation energy was extracted from the experimental H-1 NMR data.