Abstract-A model is presented by means of which the enthalpy (h) and density (p) of the products of hydrocarbonair combustion can be calculated as functions of pressure (P), temperature (T). and equivalence ratio (φ) for a given C: H ratio of the fuel, and N: 0 ratio of Ihe oxidizer. By direct differentiation, the partial derivatives of h and p with respect to P, T, and φ can also be calculated. Results of calculations using this model are compared with results from a detailed thermochemical equilibrium calculation incorporating over 40 species, and it is shown that accuracy of the approximate model is acceptable over most conditions of interest in typical combustion calculations for a range of practical fuels. The principal advantage of this method is computational speed; it thus has obvious utility in, for example, internal combustion engine performance calculations.