The absorption spectra at 25 °C. of aqueous solutions of mineral acids (HCl, HBr, HNO3, HClO4, H2SO4, H3PO4) and of some of their acid salts in various concentrations were measured in the infrared from 2 to 25 µ. In all cases three broad bands were present at 1205, 1750, and 2900 cm.−1 arising from the H3O+ ion. Cooling the samples, and even supercooling them down to liquid air temperature, produced no major changes in the spectra. The bands of the D3O+ ion were also found at 960, 1400, and 2170 cm.−1 in solutions of DCl in heavy water. Thus, the existence of discrete hydronium ions in the liquid state, with average life longer than 10−13 second, is confirmed. Infrared spectra provide a means of estimating the extent of ionization of strong acids in solution.