The structure of aqueous potassium nitrate (KNO3) solution in the concentration ranges 1.6-7.0 mol/L was studied by Monte Carlo computer simulation at 333 K. The cation (K) and solvent water molecules are regarded as the charged and dipolar rigid hard sphere, respectively, and the anion (NO3) is treated as an equivalent hard sphere with diameter equal to the Pauling diameter or a four-site rigid model in order to investigate the effect of the anionic structure on the solution structure. It is found that there is an abrupt decline in the pressure with the increase of the ion concentration for both anion models. There is a rather high degree of ionic association, and at the high ion concentration, a more complex ion configuration (such as dimer, triplet, and solvent separated cation-anion pairs with more compact structure) can be observed in the systems presented. With decreasing solution concentration, the anionic hydration number varies from 3.6 to 7.1 for the hard sphere and 3.2 to 4.7 for the four-sites model. The later is in good agreement with time-of-flight neutron diffraction results. (c) 2004 Elsevier B.V. All rights reserved.