Infrared spectra of sulfuric acid and mixtures of sulfuric acid with the mono- and dideuterated species isolated in argon matrices at 5 K were obtained in the region 4000–180 cm−1. The spectra were assigned in terms of C2 symmetry and compared with the results of the ab initio calculations. Our previous mid-infrared assignment is augmented and all the 15 normal modes of the argon isolated H2SO4 monomer are now assigned for the first time. Thermodynamic properties are calculated based upon these vibrational frequencies and structural parameters. Bands attributed to the deuterated water complex species support previous assumptions in regard to bonding and structure.