The errors in measurement employing quotient pyrometry are a function of the temperature and the effective wavelengths used. They are generated by the spectral emission coefficients of the surface of the test specimen and the spectral transmissivity of the media in the beam path. Temperature-independent deviations up to −30 K have been found for various imaging lenses using calibrated tungsten strip lamps. The influence of metal vapors over the melt was determined by spectral photometric measurements over the surface of the melting charge. No selective behavior was found for cadmium, tin, indium, nickel, chromium, iron, titanium, manganese, silicon, and stainless steel (material No. 1.4550). Measurements of the freezing points of silicon, iron, manganese, nickel, and chromium were carried out to determine the influence of the spectral emission coefficients of the surfaces of the melting charges. The ratios of the emission coefficients for wavelengths of 500 and 580 nm were below unity for all these metals, i.e., the temperature values indicated were above the actual freezing points. Measurements on corium melts (55% steel, 35% UO2, 10%) Zircaloy-2) under an oxidizing atmosphere were greatly influenced by bubbles on the surface of the melting charge and by burning phenomena of ejected particles. The error brought about by the spectral emission coefficient of the corium surface cannot be indicated.