A high-pressure and high-temperature FTIR was used to study noncatalytic Beckmann and pinacol rearrangements using supercritical water (scH(2)O). Significant acceleration of Beckmann and pinacol. rearrangements can be achieved by using scH(2)O, especially near the critical point,, even in the absence of any acid catalysts. It has been demonstrated that scH(2)O acts effectively in the place of-conventional acid catalysts for both the rearrangements. The rate of pinacol rearrangement using scH(2)O is significantly larger by a factor of 28200 than that in 0.871 M HCl solution at 46.7 MPa under distillation conditions, The activation energy for me former at 25 MPa was found to be markedly reduced to about one-third of that for the latter. The accelerated rates of reaction may be attributed to a great increase in the local proton concentration around the organic reactants. In addition, the nature of scH(2)O can be adjustable to weak acidity in the near-critical region, and then it does not catalyze the pinacol rearrangement, but opens a new reaction pathway from pinacol through a completely dehydrated product and then to a Diels-Alder adduct between the dehydrated products.