Measurements of the thermal conductivity and specific heat of commercial 99.95&percent; purity annealed polycrystalline molybdenum in the temperature range of 0.1–1 K are reported. These thermal data and other published data on molybdenum and aluminum–magnesium alloys are used to calculate and compare cool-down and steady state thermal response of 1 kHz ultracryogenic resonant-mass gravitational wave detectors made of these materials. At temperatures below 100 mK, the Mo is shown to be far superior to the 5056 Al–Mg alloy with regard to temperature homogeneity in the resonant mass at the expected levels of residual heat leak. ©1997 American Institute of Physics.