In this report two approaches are presented to predict the structure and PVT behavior of associating fluids with emphasis on water. One approach is the development of equations of state based on the analytic chain association theory (ACAT). An associating fluid is assumed to be a mixture of monomers, dimers, trimers, etc., for which the composition distribution is obtained. The resulting equations are simple enough to be used for PVT calculations. The second approach is the development of an effective Kihara pair potential for water which incorporates the hydrogen bonding using the ACAT. This potential function has been used in an analytical expression to predict the first shell of the radial distribution function (RDF) for water. The expression for RDF which was initially applied to simple potential energy functions, such as the Lennard-Jones and Kihara functions satisfies the general functionality of the RDF with respect to intermolecular potential, temperature and density as well as all the limiting values of RDF at high temperature and dilute gas, and infinite separation. The effective potential parameters are determined to predict the first shell of the RDF data for water at various subcritical temperatures and densities. The predicted results for water at near-critical and supercritical conditions are shown to be in agreement with the data obtained by neutron diffraction experiments and with the simulation data. (C) 1998 Elsevier Science B.V. All rights reserved.