Molar excess volumes, V-E, molar excess enthalpies, H-E, and speeds of sound, u, of o-toluidine (i) + cyclohexane or n-hexane or n-heptane (j) binary mixtures have been determined over entire range of composition at 308.15 K. Speeds of sound data have been utilized to predict isentropic compressibility changes of mixing, K-S(E) of (i +j) mixtures. The observed V-E, H-E and K-S(E) data have been analyzed in terms of Graph theory. The analysis of VE data by Graph theory reveals that o-toluidine exists as an associated molecular entity and (i +j) mixtures contain 1: 1 molecular complex. It has been observed that VE, HE and K-S(E) values calculated by Graph theory compare well with their corresponding experimental values. The observed data have also been analyzed in term of Flory theory. (C) 2008 Elsevier B.V. All rights reserved.