The thermal conductivity of single‐crystal UO2has been measured from liquid‐helium temperature up to room temperature. The sample temperature was determined, with a helium gas bulb thermometer, and the temperature gradient was measured with Au&sngbnd;Co vs Manganin differential thermo‐couples. In the vicinity of the Ne´el temperature,TN=30.8°K, the conductivity is depressed by two orders of magnitude, from that expected for a similar diamagnetic crystal. The experimental results demonstrate that a strong phonon‐magnon coupling exists in this crystal. The far‐infrared transmission of the same UO2crystal has been measured from a frequency of 5–115 cm−1at 3°K. A grating monochromator and a Strong‐type lamellar interferometer have been used for these measurements. A sharp absorption line has been found at 17.63±0.1 cm−1. Broader absorption lines have been observed at 19.2, 23, 79, and 100 cm−1. Transmission measurements in which a 50‐kG magnetic field is applied to the sample or the sample temperature is varied indicate that impurity‐induced absorptions occur at 19.2 and 23 cm−1, and the three remaining lines can be identified with the antiferromagnetic state of UO2.