A novel technique is presented for measuring the thermal conductivity of the pyrocarbon coatings of coated particle fuel in situ. Spherical nuclear fuel particles were overcoated with tungsten, and Chromel-Constantan thermocouple wires were welded tangent to the tungsten layer 180° apart. These intrinsic thermocouples or fission couples were subjected to neutron bursts and the surface temperature responses monitored. From knowledge of particle dimensions, burst shape, and estimates of density and heat capacity, the effective thermal conductivity of the pyrocarbon coats was calculated using a finite difference approximation to the energy equation. Experiments were conducted to measure the thermal conductivity of a low-density pyrocarbon buffer coat and comparisons were made between values obtained by the fission couple method and the xenon-flash method for two dense pyrocarbon coatings. A TRISO-I particle was tested and the thermal conductivity of the buffer layer was estimated to be 0.0039 ± 0.0011 cal/(cm sec °C).