The high-resolution infrared spectrum of perchloric acid has been observed in the 700–750 cm−1region using the infrared beamline at the MAX-I electron storage ring in Lund, Sweden. The spectrum displays extensive rotational structure due to a typeaband and is assigned to ν5, the HO–ClO3stretch. Approximately 1100 transitions in H35ClO4and ca. 300 in H37ClO4have been fitted using single subband analysis, generating constants for transitions having the sameK. The origin of H35ClO4K= 3tseries is found to be 726.9971(4) cm−1. Rotationally resolved infrared line positions are now available for the identification of HClO4in the atmosphere, which may be produced by the heterogeneous oxidation of chlorine containing species in the stratosphere.