Partial radial distribution functions of binary hard sphere systems with strong size difference between the constituting atoms are calculated starting from the Percus-Yevick equation. Partial coordination numbers of nearest neighbours are defined. Empirical relations are found which give partial coordination numbers of an accuracy better than 1 % as a function of packing fraction (0.2 ±n ±0.5), size difference([sgrave]2/[sgrave]1±1.44) and composition. Introduction of pairwise interactions between nearest neighbours yields for the enthalpy of mixing approximately the same composition dependence as given by the Flory-Huggins equation, and explains why the numerical value of the “interchange energy” depends on the choice of indexing the constituents.