The infrared spectrometer that recorded spectra of the atmosphere and surface of Mars during the Mariner 6 and 7 flyby missions is described. The instrument continuously scanned the 1.9-micro to 14.4-micro spectral region at 10 see per scan. Approximately 1% spectral resolution was furnished by two rotating, circular, variable interference filters. The spectral region 1.9-6.0 micro was recorded with a PbSe detector cooled to 175 K by radiation to deep space. The spectral region 3.9-14.4 micro was modulated by a cold (175 K) tuning fork chopper and recorded with a mercury-doped germanium detector cooled to 22 K by a Joule-Thomson two-stage (N(2) and H(2)) cryostat. The total weight of the instrument was 17.4 kg (monochromator plus electronics, 11.5 kg; gas delivery system, 5.9 kg), and it consumed 11 W of power.