The infrared spectra of the complexes formed between 1-methyl, 2-pyrimidone, 3-methyl,4-pyrimidone, amino-2-pyrimidine, 1,3-dimethyl-uracil and HBr, HCl or DCl have been measured between 4000 and 300 cm−1in KBr. Complex formation brings about strong perturbations in the infrared spectra of the base. These vibrational perturbations are discussed in terms of structural data given by x-ray diffraction methods. Some vibrational modes of the bases are more sensitive to the deuteration and this fact is interpreted by different coupling between some planar vibration modes and the νNH+or νND+modes.