The highly accurate complete basis set method CBS-QB3 has been used in conjunction with the conductor-like polarized continuum (CPCM) method to predict the aqueous pKa values for the three different hydrogen atoms in the imidazolium cation. Excellent agreement was obtained with the available experimental values. The pKa for the deprotonation of imidazole was also calculated and found to be quite different from the experimental estimate. The protocol for the pKa calculation was carefully analyzed and some recommendations made about the choice of levels of theory.