Excess molar enthalpies, H-ijk(E) of o-toluidine (i) + tetrahydropyran U) + pyridine or alpha- or beta- or gamma-picoline or benzene or toluene (k) ternary mixtures have been measured over entire mole fraction at 308.15 K using 2-drop calorimeter. The experimental data have been fitted to Redlich-Kister equation to evaluate ternary parameters along with their standard deviations. The H-rjk(E) values for the studied mixtures except those of o-toluidine (i) + tetrahydropyran U) + toluene (k) are negative over entire composition range. The topology of the constituents of the mixtures has been utilized to (Graph theory) predict H-ijk(E) values of the investigated mixtures. The observed data have also been analyzed in terms of Prigogine-Flory-Patterson theory, It has been observed that H-ijk(E) values predicted by Graph theory compare well with their corresponding experimental values. (C) 2012 Elsevier B.V. All rights reserved.