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 pK_a values for the three different hydrogen atoms in the imidazolium cation. Excellent agreement was obtained with the available experimental values. The pK_a for the deprotonation of imidazole was also calculated and found to be quite different from the experimental estimate. The protocol for the pK_a calculation was carefully analyzed and some recommendations made about the choice of levels of theory.