The thermal conductance of uranium dioxide/stainless steel interfaces has been investigated in a disc-type apparatus under vacuum and with different interface gases (helium, argon, neon), and ranges of surface roughness ([11–1417] × 10 −6 cm, arithmetical mean height measured from Talysurf profile records), interface gas pressure (7–1226 mm Hg), contact pressure (0–570 lb/in 2), mean interface temperature (55–410°C), and heat flux (1–5.5 cal/s cm 2). To obtain consistent results, the contact faces of the 2-cm diameter specimens were lapped optically flat and then roughened to a controlled amount. The thermal conductance is the sum of the solid conductance through the small areas of true contact, and the conductance through the gas in the interface (thermal radiation made a negligible contribution). The experimental values obtained for the solid conductance were of the same order as that predicted by the theory of Çetinkale and Fishenden. It was found, as expected, that the solid conductance between uranium dioxide and stainless steel was very low, owing to the hardness of the materials and the poor thermal conductivity of the uranium dioxide. Predicted values of the gas conductance, based on several simple geometrical models of the roughness, and allowing for accommodation effects in collisions between the gas molecules and the surfaces, have been compared with the present experimental results and with other published data. In nearly all cases the measured conductance is within a factor two of the predicted value, which is considered to represent good agreement in view of the very wide range of variables covered by the data and the inaccuracies inherent in this type of measurement.