Molecular reorientational motions in confined water at sub- and supercritical conditions have been studied by molecular dynamics simulations. Carbon nanotubes were chosen as confining devices because their extremely narrow diameters. We employed a flexible version of the classical simple point charge potential to model water-water interactions and Lennard-Jones-type potentials to describe water-carbon forces. Molecular rotations in the confined systems are markedly faster than in water at room conditions. At temperatures above the boiling point and in all supercritical states, reorientational. motions are basically independent of the radius of the confining tube. We can distinguish two energy domains, below and above 373 K, with an activation energy of around 15 kJ/mol. (C) 2002 Elsevier Science B.V. All rights reserved.