Excess enthalpies of mixing for six possible binary combinations of solutions of NaCl, KCl, NaBr and KBr have been determined as a function of ionic strength. The experiments were performed at constant ionic strengths of 1.000 and 2.000 mol kg(-1) at 308.15 K using flow micro calorimeter in water + N,N-dimethylformamide mixed solvent system. Data do not support Young's cross square rule. Pitzer's ion interaction model has been utilized to obtain binary and triplet interaction parameters i.e. Theta(H) and psi(H). The data were also analyzed in the light of Friedman model and it is suggested that interactions between solvated ions are dictated not only by coulombic interactions but also by appreciable asymmetric effects. The deviation from Young's cross square rule is considered to arise from significant contribution of triplet interactions and preferential solvation of the ions and ion-clusters, due to incorporation of organic co-solvent in the solvation shell. (C) 2004 Elsevier B.V. All rights reserved.