Instrumentation for making localized, precise relative reflection measurements of metals and superconductors in the terahertz frequency range is demonstrated. The results can be used to determine the surface resistivity of these materials which is of particular importance to development of high‐temperature superconductors. Emission from a commerically available 1000 °C blackbody source was reflected from the materials under test and a reference reflector. The reflected signals were detected by a Schottky diode heterodyne receiver using a CO2‐laser‐pumped 214.5 &mgr;m CH2F2laser as the local oscillator. Double‐sideband bandwidth and receiver noise temperature were 2 GHz and 30 000 K, respectively. The use of a broadband incoherent diagnostic source minimizes instrumentation sensitivity to coherent interference effects. The heterodyne receiver provides for sensitive detection with good spatial and frequency resolution. Unoptimized spatial resolution of four times the diffraction limit was achieved. The potential for relative reflectivity measurement accuracy of better than 0.1% was demonstrated with 30 min measurement times though systematic errors limited actual measurement accuracy to about 0.3%.