The quadratic concentration dependence of the logarithmic activity coefficient and relative partial molar enthalpies of concentrated strong electrolyte solutions can be explained by assuming the solution to have a pseudolattice structure and taking short-range interactions into account through a hard-core potential with an attractive 1/r(6) tail. The pseudolattice model is shown to reproduce the experimental data of activity coefficients and relative partial molar enthalpies with a great degree of accuracy over a large range of concentration. The validity of the short-range potential is explicitly tested with experimental data for a number of substances. (C) 1997 American Institute of Physics.