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 directly measuring the diffuse and hemispherical-directional reflectance and transmittance over the wavelength range of 0.35–2.5 μm. The specular properties and absorptance are calculated from the directly measured quantities. 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, side, or bottom of the sphere. By appropriately orienting the sphere, the optical properties can be measured for a variety of materials such as solids, molten salts, powders, or a falling curtain of particles, all of which are relevant to high-temperature solar applications. Absolute reflectance measurements of a standard white reference material agreed to within 2% of the NBS reference curve for the material over most of the spectral range of interest. After calibration the accuracy of solar weighted optical property measurements is better than 1%.