A conductivity cell of the thick-wire variety of the hot-wire type has been employed to measure the thermal conductivity of the binary and ternary systems: He-Ne, He-Xe, Ne-Xe and He-Ne-Xe. The values are reported in each case as a function of composition at 30, 50, 70 and 90°c. These results are interpreted on the basis of the rigorous Chapman-Enskog kinetic theory using the well-known modified Buckingham exponential-six potential. Some commonly employed approximate, empirical semi-empirical procedures of computing the thermal conductivity of gas mixtures are also discussed and critically examined on the basis of these data.