Quantum yields, ϕ for the ionization of water to hydrogen and hydroxyl ions, present as transients in excess of their equilibrium concentration levels following irradiation by a pulse derived from a Q-switched laser, have been measured as a function of temperature and excitation wavelength. Irradiation wavelengths used were the ruby and neodymium laser values 0.694 and 1.06 µm, and stimulated Raman scattering shifts of these to 0.975 and 1.41 µm. ϕ(298 K) increased from 9 × 10^–9 to 9 × 10^–6 with decrease of λ from 1.41 to 0.694 µm. The reaction occurred after absorption of a single photon into the excited O—H stretching vibrational levels: the energy of the laser photon E was in all cases in excess of the activation energy for reaction, which is shown to equal ΔH^⊖ for the self dissociation of water. The expression ϕ=ϕ₀(1–ΔH^⊖/E)^s is based upon competition between reaction and relaxation, and correlates all the data with s= 8.4 ± 0.2 and ϕ₀=(1.7 ± 0.3)× 10^–4. Values of s and ϕ₀ are interpreted with a hydrogen-bonded species (H₂O)₄ as the reactive entity. The ionization of water is the first reported example of an i.r. photochemical reaction in the liquid phase.