ESDU AERO 00.01.09 gives theoretical expressions, obtained by consideration of the behaviour of a diatomic molecule, in terms of the characteristic vibrational temperature for the ratio of specific heat capacities, the specific heat at constant pressure and the specific enthalpy. The expressions are plotted for nitrogen, oxygen and carbon monoxide alongside experimental curves for the undissociated gases extracted from the literature and are seen to agree closely; for air the experimental data relate to dissociation equilibrium and the effect of pressure is shown in the range 0.01 to 1 atmosphere for the first two properties. For the other gases the effect of pressure in that range is negligible as it is for air in the case of specific enthalpy. Values of the characteristic temperature and of gas constant used are tabulated.