The effect of pressures up to 1, 447 bar on the rate of the acid-catalyzed rearrangement of pinacol bas been measured at 60～70℃ in dilute hydrochloric acid and at 25℃ in concentrated sulfuric acid. The volumes of activation in the former at 1 bar are in the range from 6.8 to 8.4 cm^3/mole, and decrease as the temperature rises. The energy and entropy of activation at 1 bar are 36.1kcal/mole and 22.3 cal/deg・mole, respectively, and both increase with increasing pressure. On the other band, the volume of activation in the latter at 1 bar is 6.1 cm^3/mole in 47.5 wt% and 5.6 cm^3/mole in 55.8 wt%' sulfuric acid. A modified mechanism for the rearrangement has been discussed in the light of these results. Consequently, it is suggested that the slow step is the migration of the methyl group of the intermediate ion (R...OH_2)^+ formed from the protonated pinacol and is not probably varied with the acidity of the solution
