Absolute pK_as of the hydroxyquinolines in aqueous solution are calculated through application of high level molecular orbital gas-phase Gibbs energies and solvation Gibbs energies using a variety of different methods. The gas-phase energies were obtained at the G3(MP2) level, while solvation energies were calculated using six different methods; the Langevin dipole (LD) and Poisson–Boltzmann (PB) methods, two semi-empirical quantum mechanical methods SM2 and SM5.42R/A, and the polarizable continuum models PCM and IEF-PCM. The pK_as obtained using the SM2, SM5.42R/A and LD solvation energies showed significantly larger deviations from experiment (unsigned average deviations of 1.09–5.33) than the PB, PCM or IEF-PCM methods (unsigned average deviations of 0.46–1.60) for the calculation for amphoteric molecules. None of the methods studied appear to be able to accurately predict solvation energies of the zwitterionic species. For the dissociation constants involving these species, large deviations from experiment were observed with all models. The LD method performed least well, showing consistently large deviations from experiment.