The composition and transport properties of CO2, CO, CH4, CO+Ar, CO+Fe have been calculated at constant volume. Except at low temperature ( T<3000 K ) with the formation of condensed species or more complex molecules, pressure increases with temperature at constant volume. For example for 1 mole of CH4 starting at 0.1 MPa and 298K it can reach 35 MPa at 20000 K. The net result is a shift to higher temperature of dissociation and ionization. The electrical conductivity ae at constant volume increases drastically relatively to that obtained at 0.1 MPa over 15000 K, in spite of the decrease of the electron density n?. This is due to the increase of neutral species ni with a much lower electron-neutral species collision cross section σei (σe is increased proportionaly to the inverse of ni*σei ) . The viscosity exhibits always a maximum when the ionization degree reaches 3% but this maximum is shifted to a higher temperature and its peak value is higher. The thermal conductivity peak due to dissociation is shifted to higher temperature and its value is reduced while the conductivity peak due to ionization, again being shifted to higher temperature, is increased in its value.