The Conductor-like Polarizable Continuum Model (CPCM) in conjunction with the radii of UAHF, UAKS, Pauling, Bondi and KLAMT was used to calculate the pK(a) values of benzo-quinuclidine series in aqueous solvent. Comparison of calculated pK(a)'s and experimental data indicates that the accuracy of the CPCM model predominantly depends on the choice of cavity models. The most accurate pK(a)'s were obtained from the use of CPCM with UAKS or UAHF cavity, producing a mean absolute deviation of 0.39 and 0.40 pK(a) units, respectively. These methods are suitable for calculating the pK(a)'s of the relatively large ammonium ions in aqueous solution. (c) 2007 Elsevier B.V. All rights reserved.