Generalized van der Waals theory is used to establish a partition function that considers the effect of density on rotational and vibrational motions in addition to translational degrees of freedom. A three-parameter equation of state is derived from this partition function. The equation of state is applicable to small, spherical (argon-like) molecules and also to large, structurally complex molecules, including polymers. Parameters are presented for three polymers and twenty-two common fluids including light hydrocarbons, heavy hydrocarbons, nitrogen, carbon dioxide, hydrogen, carbon monoxide, hydrogen sulfide, sulfur dioxide, and water. The equation of state gives good liquid densities, and vapor pressures, and fair second virial coefficients.