Osmotic coefficients were determined for the ternary system lithium chloride-tetra-H-butylammonium chloride-water over a wide range of concentrations and ionic strength fractions at 25 degrees C by utilizing a gravimetric isopiestic technique. The mean ionic activity coefficients were calculated using the Scatchard-Rush-Johnson equations. Using the theories developed by Friedman and by Leifer and Wigent, the excess free energy, Friedman＇s interaction parameters, and contributions due to pairwise, triplet, and quadruplet interactions were determined. The types and relative strengths of ionic interactions in the system containing the hydrophilic lithium ion and hydrophobic tetrabutylammonium ion are discussed.