The phase diagrams of the binary organic systems of benzidine with p-nitrophenol, m-aminophenol and resorcinol exhibit a double simple eutectic type phase diagram in each case, showing formation of one addition compound with a congruent melting point flanked by two eutectics. The values of the enthalpy of fusion of the pure components, the eutectics and the addition compounds, as determined by the DSC method, suggest that there is appreciable interaction between the two components forming the eutectic melts. Using enthalpy of fusion data, the enthalpy of mixing, entropy of fusion and excess thermodynamic functions were calculated and the results are explained with respect to interactions between the components.