Abstract It is well known that the multiplexing advantage of Fourier transform spectroscopy effectively overcomes the problem of the low intrinsic intensity of available sources in the far infrared region. With standard instrumentation it is possible, for example, to record absorption spectra below 100 cm −1 where a pseudo-black-body source emits about two orders of magnitude less flux than at 2000 cm −1 . Even in IR reflection-absorption studies on single crystal metal surfaces signal-to-noise ratios of at least 1000 are possible in the far infrared using Fourier spectroscopy. The spectral range below 1000 cm −1 is particularly interesting in the latter experiment because nearly all external adsorbate vibrations occur in this range. With the Fourier technique one has the added advantage of investigating them with almost unlimited resolution and high wavenumber precision. The measurement of the natural linewidth of adsorbate-substrate vibrations will, for example, in future give important, and at present, unavailable information on damping mechanisms. At the Fritz-Haber-Institut we have built a slow-scan Fourier IR reflection-absorption system which is currently being commissioned. First results from the adsorption of benzene on Pt(111) at 100 K show an absorption band from the monolayer at 826 cm −1 due to the out-of-plane symmetric C-H wag (v 4 ); multilayers are characterised by a very intense band at 684 cm −1 and another at 1032 cm −1 . These results are in good agreement with an earlier EELS study by Lehwald et al. (ref. 1), except that the present study delivers the natural linewidths (5, 7 and 6 cm −1 , respectively). A further band at 1238 cm −1 (halfwidth 24 cm −1 ) in the multilayers was not observed in the EELS study and can probably be identified with the v 11 + v 20 combination band (ref. 2). Investigations of the C-O stretch in the system CO/Pt(111) reveal a halfwidth (6 cm −1 ) similar to that of the C-H wag in the monolayer, indicating that the damping of vibrations by phonon creation plays only a minor role in determing the observed linewidth of the benzene mode. At 800 cm −1 this is still to be expected (ref. 3). So far, we have been unable to detect the metal-carbon vibration in the CO/Pt(111) system, indicating that it is at least a factor 20, if not a factor 50, weaker than the C-O stretch for the on-top site. Synchrotron radiation (SR) offers the possibility of combating the problem of the low intensity of conventional sources in the far infrared. As our recent measurements have shown (ref.4), the time structure of electron storage rings has no adverse effect on the resolution in Fourier transform spectroscopy. General considerations show that SR can offer considerable signal-to-noise advantages compared to a conventional source in a far infrared experiment where sample size is the throughput-limiting factor. This is due to the high brightness of SR, particularly with the new generation of dedicated sources. In IR reflection-absorption studies the optimal, near-grazing reflection geometry results in the single crystal itself becoming the throughput-limiting element. For the IR beam line at the Berlin electron storage ring BESSY and a single crystal of 10 mm the advantages of SR are already apparent at about 800 cm −1 . We are planning such experiments for the far IR range below 500 cm −1 .