[en] A dimensional analysis of the classical partition functions of vapors of ionic molecules leads to expressions for the relative values of thermodynamic properties for molecules having one type of cation and one type of anion. Relative values of standard entropies, S⁰, and of free energy functions, (G⁰--H⁰₀)/T, can be calculated with a knowledge of the cation-anion distance, the atomic masses, and the electronic partition function. The theory is not expected to be valid for energies and enthalpies. Comparisons of the theoretical predictions with data on S⁰ and (G⁰--H⁰₀)/T for a large number of diatomic molecules and their associated dimers demonstrate the accuracy of this simple calculation. Covalent molecules fit the expressions derived for ionic molecules surprisingly well indicating that the equations might be more general than implied by the model. The model should be most useful for polyatomic molecules, M/sub m/X/sub x/, where m and/or x is greater than unity. The theory minimizes the amount of spectroscopic and structural information necessary to calculate some of the thermodynamic properties since functions such as S⁰ and (G⁰--H⁰₀)/T can be calculated for molecules of a given type if the corresponding data for one member of the class are well known