Neutron diffusion parameters in water have been measured at 26.7°C with the pulsed neutron technique. The results are 210 ± 1 µsec for the neutron mean lifetime, 37,503 ± 366 cm2 sec−1 for the average diffusion coefficient, and 5116 ± 776 cm4 sec−1 for the diffusion cooling constant. From these values the thermal absorption cross section of hydrogen and the thermal diffusion length in water can be inferred to be 325 ± 2 mb and 2.83 ± 0.02 cm, respectively. With a pulsed high-intensity neutron source provided by an electron linear accelerator, neutron lifetime measurements were performed on small and large water samples with values of the geometrical buckling from 0.014 cm−2 to 0.59 cm−2. Effects of harmonic modes in the large water geometries, which were determined by measurements of the time-dependent spatial flux distributions resulting from an external pulsed source of fast neutrons, were found to be adequately predictable with simple diffusion theory.