The composition and transport properties of CO2, CO, CH4, CO+ Ar ( 50 vol%), CO+ Fe ( 50 vol%) have been calculated at constant volume assuming local thermodynamic equilibrium ( LTE). Except at low temperature ( T < 3000 K), when the formation of condensed species or more complex molecules can occur, pressure increases with temperature at constant volume. For example, for 1 mol of CH4 starting at 0.1 MPa and 298 K the pressure can reach 40 MPa at 20,000 K. The consequence is a shift to higher temperatures of dissociation and ionization. The electrical conductivity se at constant volume increases drastically relative to that obtained at 0.1 MPa over 15,000 K, in spite of the decrease of the electron density ne This is due to the increase in the neutral species density. n(i), with a much lower electron-neutral species collision cross section sigma(ei) (sigma(e) is inversely proportional to ni, sei). The viscosity always exhibits a maximum when the ionization degree increases over 1-30%, but this maximum is shifted to higher temperatures and its peak value is higher. The thermal conductivity peaks due to dissociation and ionization are shifted to higher temperatures and their values are reduced compared to those obtained at constant pressure.