In order to study proton transfer and hydrogen bonding interactions in acid/base complexes, the equilibrium structure of a stoichiometric pyridine-methanesulfonic acid complex has been investigated using density functional theory with different functionals and compared with second-order Moller-Plesset perturbation theory and restricted Hartree-Fock calculations. The basis sets range from 6-31G to 6-31++G(d,p). Additionally, interaction energies and equilibrium geometries were calculated with 6-311++G(d,p) basis set, while aug-cc-pVDZ was used only with BLYP method. In the gas phase, the optimized geometry shows hydrogen bonding without proton transfer between the molecules. The polarized continuum model suggests that embedding the complex in a dielectric medium enhances the proton transfer. The role of added water was studied explicitly by optimizing the geometry in the presence of 1 or 2 water molecules. In such cases the proton is observed to transfer. The crystalline structure of the pyridine-methanesulfonic complex was also determined experimentally using x-ray diffraction, showing that proton transfer occurs. In this case, also geometry change which has been induced by the crystal packing may play a role. In conclusion, both the dielectric medium and the configuration of the complex have a pronounced effect on the potential energy surface experienced by the hydrogen in the bond between an aromatic base and a strong acid.