The densities, rho, speeds of sound, u of {1-ethyl-3-methylimidazolium tetrafluoroborate (i) + pyridine or alpha- or beta- or gamma-picoline (j)} at T/K = (293.15, 298.15, 303.15, and 308.15) and excess molar enthalpies, H-E of the same set of mixtures at T/K = (298.15) have been measured over entire mole fraction range using DSA-5000 and 2-drop microcalorimeter. Excess molar volumes, V-E and excess isentropic compressibilities, kappa(E)(S) values have been predicted by utilizing the measured densities and speeds of sound data. It has been observed that V-E, H-E, and kappa(E)(S) values for the studied mixtures are negative over entire composition. The connectivity parameter of third degree of a molecule, (3)xi (which in turn depends upon its topology) have been applied to predict (i) state of components of ionic liquid mixtures in their pure and mixed state; (ii) nature and extent of interactions existing in mixtures; and (iii) V-E, H-E, and kappa(E)(S) values. The analysis of V-E data in terms of Graph theory (which deals with topology of a molecule) suggest that while 1-ethyl-3-methylimidazolium tetrafluoroborate is characterised by electrostatic forces of attraction and exist as monomer; alpha- or beta- or gamma-picoline exist as associated molecular entities. Further, (i + j) mixtures are characterized by interactions between nitrogen and florine atoms of 1-ethyl-3-methylimidazolium tetrafluoroborate with nitrogen and carbon atoms of pyridine or isomeric picolines to form 1:1 molecular complex. The IR studies also support to this view point. The V-E, H-E, and kappa(E)(S) values predicted by Graph theory compare well with experimental values. (C) 2012 Elsevier Ltd. All rights reserved.