A first order theory is developed for the calculation of Helmholtz energy differences. The theory is easy to use and requires only the well−known hard sphere pair distribution function and the pair intermolecular potential function for the species under considerationTriacC%%%%%ermolecular potential Lennard−Jones fluids and mixtures of Lennard−Jones fluids indicate that the theory has a domain of validity limited to molecules with nearly equal ε and σ values.