A state-of-the-art, high-temperature integrating sphere spectrometer has been fabricated and characterized. The liquid-cooled sphere is combined with a furnace designed for a maximum specimen temperature of 1200°C. The spectrometer is capable of measuring the spectral, diffuse, and hemispherical reflectance and transmittance over the wavelength range of 0.35-2.5 micrometers. Chopped white light from a high pressure xenon arc-lamp is introduced into the sphere, alternately reflected from the sphere wall and reflected from or transmitted through the specimen, spectrally resolved by a monochromator, and focused onto a two-color Si/PbS detector. Data are sampled at wavelengths corresponding to equal energy bands of the terrestrial solar distribution and are graphically displayed. Data acquisition and hardware module commands are computer controlled. The sphere and light collecting optics can be rotated about the horizontal optical axis of the monochromator so that the specimen port is at the top (0°), side (90°), or bottom (180°) of the sphere. This allows optical characterization of a variety of materials relevant to high-temperature solar energy applications to be carried out by rotating the instrument into a desired orientation. For example, in the 0° ("look up" at the sample) position, reflectance and transmittance of molten salt in a furnace-heated cell can be measured without salt vapor entering the sphere. At 90°, a vertical wall of falling sand particles can be characterized. In the 180° ("look down") position, the optical properties of solids such as ceramics, metal alloys, or powders can be ascertained.