Miniature planar IR waveguides of thickness 30-50 .mu.m, consisting of 12-mm long, 2-mm wide strips of Ge supported on ZnS substrates and tapered quasi-tapered waveguides, tapered from a thickness of 1 mm at the ends to a minimum of 1-100 .mu.m at the center, are disclosed. The surface sensitivity is increased as a function of incidence or bevel angle. The tapered waveguide improves the efficiency of the optical coupling both into the waveguide from an FTIR spectrometer, and out of the waveguide onto a small-area IR detector. The tapering makes it possible to dispense with using an IR microscope couple light through the waveguide, enabling efficient coupling with a detector directly coupled to an immersion lens. This optical arrangement makes such thin supported waveguides more useful as sensors, because they can be made quite long (e.g. 50 mm) and mounted horizontally. Furthermore, even with a 20-.mu.m.times.1-mm cross section, sufficient throughput is obtained to give signal/noise ratios in excess of 1000 over most of the 1000-5000 cm.sup.-1 range, with just 2 min of scanning at 8 cm.sup.-1 solution.