A new equation of state based on the concept of perturbed hard chain theory and the hard-convex body equation of state has been successfully developed for nonpolar compounds. The equation can predict the thermodynamic properties (density, enthalpy departure, and vapor pressure) of a wide range of pure fluids from small, spherical (argon-like) molecules to large structurally complex compounds. For normal paraffins, the size parameter (hard-core volume) is related to the measurable Van der Waals volume given by Bondi (1968). For most other compounds, it is related to the critical volume. The shape parameter values reflect the structure and degree of acentricity of the compound of interest. The equation has been extended to polar and associating compounds by using the effective angle-averaged potential model. For polar compounds, a fourth parameter is required. The equation has been extensively tested for polar (dipolar and quadrupolar) and hydrogen-bonding compounds. The results show that the equation has promise for use for systems occurring in the natural gas, petroleum and synfuels industries.