Molar excess volumes, V-ijk(E), and speeds of sound, U-ijk, of o-toluidine (i) + toluene (j) + o- or p-xylene (k) ternary mixtures and speeds of sound, U-ij, of o-toluidine (i) + toluene or o- or p-xylene (j) binary mixtures have been determined as a function of composition at 308.15 K. Speeds of sound data have been utilized to evaluate excess isentropic compressibilities for binary (i + j) and ternary (i + j + k) mixtures. Molar excess volumes, V-ijk(E), and excess isentropic compressiblities, (K-S(E))(ijk) data have been fitted to the Redlich-Kister equation to calculate ternary adjustable parameters and standard deviations. Flory theory for binary mixtures has been extended to predict V-ijk(E) and (K-S(E))(ijk) for the studied ternary mixtures. It has been observed that theory correctly predicts the Sign of V-ijk(E) values for o-toluidine (i) + toluene (j) + o-xylene (k) and (K-S(E))(ijk) values for o-toluidine (i) + toluene (j) + p-xylene (k) ternary mixtures.