Infrared (IR) spectra of sulfadimethoxine (SDMX), sulfamethoxazole (SMX) and sulfanilamide (SD) have been simulated by a set of the semi-empirical methods supplied by the HyperChemTM package. Before the IR simulations, the initial geometries of the molecules were built by means of X-ray diffraction data and standard parameters and full geometry optimizations were performed with the unrestricted Hartree-Fock basis, Polak-Ribiere conjugate gradient algorithm. It was found that IR spectra simulated by the semi-empirical method MIND03 gives the best match to the observed spectra and also this method provides the best linearity between calculated and experimental wave numbers (with a correlation coefficient of 0.99780). The certain assignment of the most useful vibrational modes of sulfonamides were determined by using semi-empirical methods. At the same time, none of the methods is able to predict infrared intensities and a spectral intensity pattern.