The high-resolution infrared spectrum of perchloric acid has been observed in the 700-750 cm-1 region using the infrared beamline at the MAX-I electron storage ring in Lund, Sweden. The spectrum displays extensive rotational structure due to a type a band and is assigned to nu5, the HO-ClO3 stretch. Approximately 1100 transitions in H35ClO4 and ca. 300 in H37ClO4 have been fitted using single subband analysis, generating constants for transitions having the same K. The origin of H35ClO4 K = 3t series is found to be 726.9971(4) cm-1. Rotationally resolved infrared line positions are now available for the identification of HClO4 in the atmosphere, which may be produced by the heterogeneous oxidation of chlorine containing species in the stratosphere. Copyright 1998 Academic Press.