An experimental study of the proton spin-lattice relaxation time T1 has been made for H2O over the temperature interval from -16 to 145°C. It is found that after correction for spin-rotational interaction, the experimental T1 behavior can be represented by the double exponential form of the rate expression used to treat the relaxation for the quadrupolar nuclei 2H and 17O in water. The oxygen-17 data are used to calculate the intramolecular contribution to the proton T1. The activation energies E1=9.4± 0.8 kcal/mole and E2=3.6± 0.1 kcal/mole for the two contributions to the intermolecular relaxation are in sufficiently good agreement with those for the intramolecular relaxation to indicate that the relaxation mechanism is the same in both cases. This mechanism involves two processes. The data indicate the process dominant at high temperature can be described as a rotational diffusion where the amplitude of angular motion increases with increasing temperature.