Dense UN specimens for property tests were fabricated primarily by an isostatic hot-pressing technique in which consolidated powder sealed in a refractory metal container was subjected to a 10,000-psi external gas pressure at 1480 to 1540 deg C for 3 to 4 hr. The thermal conductivity of uranium mononitride increased from 0.04 cal/(cm)(sec)( deg C) at 200 deg C to 0.06 cal/ (sec)(cm/sup 2/)( deg C) at 1000 deg C, which by an integrated thermal- conductivity criterion, makes it some eight times more effective than UO/sub 2/ in transferring heat. Thermal-expansion coefficients of 9.0 x 10/sup -6/ per deg C and 9.9 x 10/sup -6/per deg C over the ranges 20 to 800 deg C and 20 to 1600 deg C, respectively, were measured. UN retained about 80% of its room- temperature hardness at 1100 deg C. Above this temperature, the hardness dropped rapidly with increasing temperature. The specific heat is cal per g mole can be conveniently described by the equition C = 13.32 + 1.19 x 10/sup -3/ T --2.10 x 10/sup 5/ T/sup -2/ over the range 273 to 142 3 deg K. Electrical conductivity dropped with increasing temperature a metallic characteristic. Anomalies were noted in attemptsmore » to measure the hydiogen solubility and the resistance to water corrosion. UN is apparently not attacked by NaK within 1500 hr at temperatures up to 480 deg C. (auth) « less