A~tract-. The photochemistry of three water soluble thioxanthone photoinitiators is studied in water using absorption, luminescence (fluorescence and phosphore~ence) and conventional microsecond flash photolysis techniques. Longest wavelength absorption maxima and extinction coefficients are similar to those of the oil soluble types studied earlier. The fluorescence and phosphorescence properties, on the other hand, differ significantly. In this case, certainly for two of the initiators, high fluorescence and very low phosphorescence quantum yields are observed, the former being effectively quenched by the addition of a tertiary amine, diethylmethylamine. End-of-pulse transient absorption spectra show the presence of a weakly absorbing semiquinone type radical at ~ 340 nm formed by hydrogen atom abstraction. In the presence of the tertiary amine, transient formation due to the neutral ketyl radical is significantly enhanced coupled with the formation of a new absorption band at longer wavelengths, 480 510 nm, due to the radical anion formed by electron transfer. In the presence of oxygen, particularly with the tertiary amine. transient formation due to hydrogen atom and electron abstraction is little affected and is consistent with the luminescence data, suggesting that these types of photoinitiators initiate free radical formation via the singlet state. The significance of these results, in contrast to those for the oil soluble types, is discussed.