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We prepared eight samples of Mo/X and W/X (X = BN:O, [B.sub.4]C:O, Si, and C) multilayers by magnetron sputtering. Analyses of x-ray photoelectron spectroscopy for the boron nitride and the [B.sub.4]C layers showed the concentration of O to be nonnegligible. We have evaluated the thermal stability by measuring soft-x-ray specular reflectances before and after thermal annealing occurs at temperatures as high as 700 [degrees] C. The results suggest that the thermal stability depends largely on the inclusion of low-density materials and not on the type of metal. Of the four low-density materials studied, BN:O is thermally the most stable, and the Mo/BN:O multilayer, the most stable among the eight samples, shows stability as high as 700 [degrees] C.