The equilibrium composition and thermodynamic and transport properties of argon/carbon and helium/carbon mixtures are calculated in the temperature range 300-20,000 K. The curves for the composition of mixtures of 50% carbon in argon or helium are shown for a pressure of 1.33 x 10(4) Pa. The calculations for the heat capacity at constant pressure (C-p) and transport coefficients are validated with other studies, for the cases or pure argon and pure helium al a pressure of 10(5) Pa. The properties of mixtures with various proportions of carbon in argon and helium are calculated. Results are presented at pressures of 10(5) and 1.33 x 10(4) Pa, typical of reactors far the synthesis of fullerenes and nanotubes. It is observed that the properties of carbon and mixtures of carbon with a buffer gas (argon or helium) are very different from those of the buffer gas, thus the need to consider this effect in simulations. In general, the mixtures follow trends intermediate to those of the pure gases from which they are composed except for the thermal conductivity which shows a deviation from this tendency in the region between 11,500 and 19,000 K for argon/carbon mixtures and between 8,000 and 12,000 K for helium/carbon mixtures. Also, the electrical conductivity of mixtures of low carbon concentration is very close to that of pure carbon. A datafile containing the transport properties of mixtures for pressures between 10(4) and 10(5) Pa is available free of charge from the authors.