Energy-resolved, competitive threshold collision-induced dissociation (TCID) methods are used to measure the gas-phase acidities of phenol, 3-methylphenol, 2,4,6-trimethylphenol, and ethanoic acid relative to hydrogen cyanide, hydrogen sulfide, and the hydroperoxyl radical using guided ion beam tandem mass spectrometry. The gas-phase acidities of Delta H-acid(298)(C6H5OH) = 1456 +/- 4 kJ/mol, Delta H-acid(298)(3-CH3C6H4OH) = 1457 +/- 5 kJ/mol, Delta H-acid(298)(2,4,6-(CH3)(3)C6H2OH) = 1456 +/- 4 kJ/mol, and Delta H-acid(298)(CH3COOH) = 1457 +/- 6 kJ/mol are determined. The O-H bond dissociation enthalpy of D-298(C6H5O-H) = 361 +/- 4 kJ/mol is derived using the previously published experimental electron affinity for C6H5O, and thermochemical values for the other species are reported. A comparison of the new TCID values with both experimental and theoretical values from the literature is presented.