Dynamics of supercritical water was studied by quasi-elastic neutron scattering. Two series of measurements, re-spectively, at constant temperature of 673 K and constant density of 0.578 g/cm 3 were carried out. Experimental data were fitted properly by the jump-diffusion model. The self-diffusion coefficient, typically in the interval 3.71–5.21 Â 10 À8 m 2 /s, depends both on temperature and density. The mean residence time is, however, fairly constant at 0.30 ps in all cases. From the temperature dependence of self-diffusion coefficient which is of Arrhenius type the activation energy E a ¼ 2:25 kcal/mol is being estimated. Ó 2003 Elsevier Science B.V. All rights reserved.