The density, rho(ijk) and speed of sound, u(ijk), ternary {o-toluidine (i) + tetrahydropyran (j) + alpha-, beta-, or gamma-picoline (k)} mixtures have been measured over the entire composition range at T = (298.15, 303.15, and 308.15) K. The excess molar volume, V-ijk(E) and excess isentropic compressibility, (K-S(K))(ijk) values of the mixtures have been predicted by using the experimental results. The observed thermodynamic properties have been fitted to Redlich-Kister equation to determine ternary adjustable parameters and standard deviations. The V-ijk(E) and (K-S(E))(ijk) values have also been analyzed in terms of (1) Graph theory and (2) Prigogine-Flory-Patterson theory (PFP). It has been observed that V-ijk(E) and (K-S(E))(ijk) data determined by Graph theory compare well with experimental values. (C) 2011 Elsevier Ltd. All rights reserved.