A series of five trifluoroacetophenones and five phenyl trifluoromethyl carbinols have been synthesized and their ionization constants in water measured. They are the unsubstituted,p-methoxy,p-methyl,m-bromo, andm-nitro compounds. The ketones exist as hydrates in aqueous solution and each is about two pKunits stronger than the corresponding alcohol. The acidities of both series are correlated by the Hammett relation with ρ values of 1.11 and 1.01 for the ketone hydrates and alcohols respectively. The acid strengths vary from a pKaof 9.18 form-nitro-α,α,α-trifluoroacetophenone to 12.24 forp-methoxyphenyl trifluoromethyl carbinol.