A nondecaying weak luminescence of intensity 1.5 ± 0.1 photons ${\rm sec}^{-1}\ {\rm ml}^{-1}$ has been observed from purified water exposed to ambient ionizing radiation. The structured emission spectrum and the observed quenching by dissolved oxygen, indicated that a substantial portion of the luminescence was non-Cerenkov emisson. Emission peaks were observed at 275 and 370 nm, and a broad emission band was observed between 400 and 700 nm. The peak at 275 nm was attributed to emission by excited triplet water molecules, and the peak at 370 nm was assigned to emission by OH radicals in the ${}^{2}\Sigma {}^{+}$ state.