A sapphire fiber-optic thermometer which was developed at the National Bureau of Standards (NBS) has been evaluated up to 1600 K. The thermometer operates by emitting radiation from a thin-film metal cup on a sapphire light pipe which is immersed in the hot gas to be measured. Characteristic blackbody emission of the thermometer is transmitted through a controlled optical path to a photodiode. Simultaneous measurements at two wavelengths (596 and 702 nm) were made, and their ratio was used to derive two-color radiation measurements. Several tip geometries and platinum thin-film lengths were investigated to verify blackbody conditions. A reference platinum platinum-rhodium thermocouple was used for calibration and comparison, and a discussion of errors in the two-color ratio method is presented.