The dramatic changes which occur in the properties of water upon exceeding its critical point lead to dramatic differences in the behavior of dissolved organic solutes. In this paper, the large decreases in the self-ionization and dielectric constants of deuterium oxide which accompany exceeding the critical temperature lead to large increases in the equilibrium constants for the reactions of very weak organic acids with deuteroxide ion. The enhanced acid-base reaction is monitored by the incorporation of deuterium into the organic molecules via reaction with deuteroxide ion. Successful deuterium exchange is demonstrated for a variety of organic substance having pK(a) values up to 50. Nearly quantitative exchange of hydrogen isotopes is obtained by reaction with 0.016 M NaOD solution for 10 min at 400 degrees C at approximately 300 bar pressure for molecules having pK(a)’s up to approximately 43 while longer heating times and more concentrated base solutions allow deuteration of acids having pK(a)’s up to 50. The terminal methyl group is found to be the most acidic carbon of a typical alkyl chain and an unexpected alternation of acid strength along the alkyl chain of dibutylamine is observed.