Thermal conductivity measurements were performed to determine the characteristics of hollow glass microspheres as an insulating material and as an opacifying agent for other insulations. The experiments were carried out with a radial flow heat transfer apparatus especially designed to suppress extraneous heat transfers, both internal and external to the heated section, and to provide uniform temperatures on the bounding surfaces. Three types of microsphere insulations were investigated, differing in bulk density and in the presence or absence of an aluminizing coating. The thermal conductivity of the microsphere insulations was found to be about one and a half times that of stagnant air over a wide temperature range. Additional experiments, involving the use of an opacifier (powdered silicon), demonstrated that radiative transfer has a minor effect on the thermal conductivity of microsphere insulations. This finding was corroborated by the fact that the high-temperature conductivity of the aluminized microspheres was not appreciably different from that of the uncoated microspheres. Another set of experiments was performed in which microsphere insulation was added to opacify silica aerogel, a fine powder insulation that is markedly affected by radiative transfer. The presence of the microspheres brought about reductions in conductivity of almost a factor of two at an optimum mixture ratio of the constituents. Furthermore, it was found that the conductivity of such a mixture was lower than that of either constituent, thereby illustrating their synergistic interaction.