For maximum effectiveness of solar-thermal conversion, a solar absorber must be a low emitter of thermal radiation. Thin film absorber stacks which possessed such spectrally selective characteristics were developed by the Bureau of Mines. The stacks consisted of zirconium suboxide or subnitride absorber films on a reflective film of silver or copper. The absorber films were prepared by reactively sputtering zirconium in argon with small amounts of oxygen, nitrogen, or both, or by oxidation of zirconium films in air. The solar absorptance and the thermal emittance were calculated from reflectance data. The solar absorptance, a(s), ranged from 0.60 to 0.93 and the emittance, e(t), at 600 k, ranged from 0.024 to 0.28. Ratios of a(s)/e(t) extended from about 2 to 28. The stacks appear relatively stable under vacuum at temperatures up to about 600 k.