We have re-examined the infrared absorption between 4000 and 800 cm−1 of aqueous solutions of the four hydrogen halides at concentrations up to saturation, and under better experimental conditions than heretofore. The new spectra confirm definitely our previous assignment of the three broad bands around 2900, 1730, and 1200 cm−1 to fundamental vibrations of the H3O+ ion. Other proposed interpretations are shown to be untenable; in particular that of Ackermann, based on alleged similarities in the spectra of strong acids and bases. In hydrofluoric acid, hydrogen-bonded ion pairs are responsible for the shifts of the H3O+ frequencies. There is no evidence in the spectra for higher species, such as H5O2+. We conclude that the H3O+ ion has an appreciably longer lifetime in concentrated aqueous acids than in water. It also forms much stronger hydrogen bonds than H2O because of its ionic charge.