It is well known that the existing theories of thermodynamics have not been capable of predicting the thermodynamic properties of associating fluids with reasonable accuracy. To improve the applicability of theories for such compounds, a statistical polydisperse theory of characterizing the molecular association effects has been developed. By the application of the theories of polydisperse fluids, it has become possible to develop accurate and analytic equations of state for associating fluids. In order to demonstrate the reliability of the theory it is applied for four different kinds of associating fluids; namely water, ammonia, methanol and hydrogen sulfide over wide ranges of temperature and pressure with success.