The pK values for xanthone protonation in the S0, S1 and T1 states have been determined by direct measurement and by Forster Cycle calculation. Both methods for determining pK give the order for xanthone of pK(T1) > pK(S1) > pK(S0). A study of benzophenones and acetophenones showed this pK order to be a common one for aromatic carbonyl compounds. The pK order for these compounds has been explained in terms of the type of transitions involved and the S1-T1 splitting of these transitions. In the case of substituted aromatic carbonyl compounds the pK order reflects the effect of substitution on the lowest S0-S1 and transitions. Results are also presented on the effect of inorganic anions solvenli and temperature on xanthone fluorescence. Spectral details are reported for several xanthones, benzophenones, acetophenones, and anthraquinones.