Energy-resolved competitive collision-induced dissociation methods are used to measure the gas-phase acidity of phenol relative to hydrogen cyanide. The competitive dissociation of the [C6H5O-H-CN](-) complex into C6H5OH + CN- and C6H5O- + HCN is studied using a guided ion beam tandem mass spectrometer. The reaction cross sections and product branching fractions are measured as a function of collision energy. The enthalpy difference between the two reaction channels is found by modeling the reaction cross sections near threshold using RRKM theory to account for the energy-dependent product branching ratio and kinetic shift. From the enthalpy difference, the phenol gas-phase acidity, Delta(acid)H(0)(C6H5OH) = 1448 +/- 8 kJ/mol, is determined relative to the established literature value of hydrogen cyanide, Delta(acid)H(0)(HCN) = 1462.3 +/- 0.9 kJ/mol. We then derive Delta(acid)H(298)(C6H5OH) = 1454 +/- 8 kJ/mol and the bond dissociation energy of D-298(C6H5O-H) = 359 +/- 8 kJ/mol.