The equilibrium temperature of a coated Langmuir probe in space is determined by the optical properties of the coating and the geometry of the probe. In this investigation we report on the optical properties of graphite- and TiN-coated aluminium and titanium. Earlier results have shown that a TiN-coated sphere is preferred as a Langmuir probe for the Cassini satellite bound to Saturn owing to the hardness, chemical inertness and photoelectric stability of the TiN coating. The TiN coatings were prepared by high temperature nitriding of a titanium alloy in pure nitrogen gas. An epoxy-based graphite dispersion (DAG 213) was used to apply the graphite coating. The integrated total solar absorptance alpha(s) was calculated, for a spherical and a cylindrical probe, using measured total reflectance. Total hemispherical emittance epsilon(H)(T) of spherical samples was measured using a calorimetric method and compared with infrared reflectance measurements from 2 to 30 mu m. The ratio alpha(s)/epsilon(H) for graphite was determined to be 0.95-1.11 for spherical and cylindrical probes in the temperature interval 90-380 K. For TiN the ratio alpha(s)/epsilon(H) is 2.64-4.64 for temperatures between 130 and 490 K. Equilibrium temperatures for spherical and cylindrical probes were estimated for solar constants at Venus, Earth and Saturn.