Tungsten and tungsten-molybdenum alloys (95: 5 w/w) with a three-dimensionally ordered macroporous (3DOM) structure were prepared by infiltration of poly(methyl methacrylate) colloidal crystals with acetylated peroxotungstic acid and acetylated peroxomolybdic acid as solution precursors. In contrast to previous preparations, by using these precursors, it was possible to maintain the original metal ratio in the product obtained after hydrogen reduction, thereby permitting better control over the product composition. To evaluate the suitability of these metallic photonic crystals for thermal emission processes, the thermal stability of 3DOM W and 3DOM W/Mo was investigated by SEM analysis for samples treated ex situ in nitrogen at 800-1000 °C and by TEM analysis with in situ heating to follow real-time changes in high vacuum between room temperature and 1000 °C. Grain coarsening and distortion of the structure that starts in 3DOM W at 800 °C in a nitrogen atmosphere could be delayed to higher temperatures by heating the sample in high vacuum and by alloying with 5 wt% Mo. In the pure metal, structural changes were first seen for feature sizes below ca. 30 nm, whereas similar changes occurred in the alloy only for much smaller features below ca. 3 nm. The TEM analyses also provided indirect evidence for reduced stresses in the photonic crystal during thermal treatment.