AbstractIrradiation of trifluoromethylphenols (three isomers) and ‐naphthols (eight isomers) in water solution yields the corresponding hydroxybenzoic and naphthoic acids, respectively. The reaction mechanism has been investigated by means of flash photolysis and quenching experiments. It is proposed that the photohydrolysis is initiated in the lowest excited singlet state by a heterolytic CF bond cleavage. The observed photochemical and thermal reaction rates as well as the shifts of the lowest electronic transitions upon CF3‐substitution compare well with predictions from semiempirical π‐electron calculations (PPP SCF CI). A rationale for the correlation between excited state charge densities and photochemical reactivity is given. pK values for the ground state and first excited singlet state are