Well established, simple NMR techniques are applied to the in situ study of hydrogen exchange reactions in supercritical aqueous solutions and the supercritical alcohols methanol and ethanol. In the case of the rapidly exchanging hydroxyl protons of alcohols and water, changes in the exchange rate are evidenced by the collapse of chemical shift differences (in solutions) and the collapse of J-multiplets. We show that the hydrogen exchange rate can be adjusted by changing pressure (density) at a constant supercritical temperature. Higher pressure at constant temperature increases the exchange rate, indicating that hydrogen exchange at supercritical conditions proceeds via charged intermediates. The pure alcohols at temperatures above 200 degrees C exhibit exchange of hydroxyl protons at a rate of 5 s(-1) already at low, gas-like densities. However, the hydrogen exchange rate in the pure and aqueous alcohols is found to be very sensitive to the presence of impurities. The much slower deuteration of aliphatic and aromatic protons in 4-ethylphenol by supercritical D2O was monitored in situ from the time dependence of the resonance intensities. Suitable pressure-temperature conditions are identified for the purpose of selective deuteration of 4-ethylphenol. The corresponding reaction rates are reported.