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 308.15K. Speeds of sound data have been utilized to predict isentropic compressibility changes of mixing, κSE of (i+j) mixtures. The observed V E , H E and κSE data have been analyzed in terms of Graph theory. The analysis of V E 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 V E , H E and κSE values calculated by Graph theory compare well with their corresponding experimental values. The observed data have also been analyzed in term of Flory theory.