The thermal conductivity of synthetic char particles was measured in an electrodynamic chamber, and modelled by a microstructure model represented by a network of randomly distributed and interconnected microcrystals. Reaction proceeds over the microcrystal surface with preferential consumption at the narrow joints between adjacent microcrystals, leading to the breakage of the joints. Subsequently, either of the following two processes can occur: 1) restoration of the joints due to intercrystal attraction forces, 2) coalescence of microcrystals. The network of the microcrystals can be viewed as a complex connection of thermal resistances. The effective thermal conductivity of the micromedium is represented in terms of: 1) the effective thermal conductivity related to a single microcrystal, and 2) the number of microcrystals intersecting a unit surface area. The change of these characteristics in the course of oxidation is affected by: 1) consumption of carbon on the internal surface, 2) coalescence of microcrystals, 3) activation of intercrystal joints, and 4) graphitization at high conversion. The calculated thermal conductivity of synthetic char (Spherocarb) is in good agreement with the experimental data. The results show that the change in the thermal conductivity occurs primarily due to structural transformation in the micromedium, and thai the role of the overall porosity is not dominant.