The densities, rho, speeds of sound, u, and molar heat capacities, C-P, of 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (i) + 1-butyl-3-methylimidazolium tetrafluoroborate (j); 1-butyl-3-methylimidazolium tetrafluoroborate (i) + 1-ethyl-3-methylimidazolium tetrafluoroborate (j) and 1-butyl-2,3-dimethylim idazolium tetrafluoroborate (i) + 1-ethyl-3-methylimidazolium tetrafluoroborate (j) mixtures at temperatures (293.15, 298.15, 303.15, 308.15) K and excess molar enthalpies, H-E, for the same mixtures at temperature (298.15) K have been measured over entire range of mole fraction under atmospheric pressure. Using experimental data, excess molar volumes, V-E, excess isentropic compressiblities, kappa(E)(S), excess molar enthalpies, HE and excess molar heat capacities, C-P(E) have been determined. The V-E, kappa(E)(S), H-E and C-P(E) data have been fitted to Redlich-Kister equation to estimate the binary adjustable parameters and standard deviations between experimental and calculated values. The topology of the constituent molecules (Graph theory) has been utilized to predict V-E, kappa(E)(S), HE and C-P(E) data of the studied mixtures. The comparison of V-E, kappa(E)(S), H-E and C-P(E) values determined by Graph theory with their corresponding experimental values lends additional support to the proposed molecular entities existing in mixtures along with various processes involved in the (i + j) mixtures formation. Further, IR studies and quantum mechanical calculations support the presence of proposed molecular entities in the studied mixtures. (C) 2016 Elsevier Ltd.