Effect of N-N'bis(2-pyridylmethylidene)-1,2-diiminoethane (BPIE) Schiff base on thermodynamic properties of ionic liquid, 1-hexyl-3-methylimidazolium bromide ([HMIm]Br) in N,N-dimethylformamide (DMF) solutions were determined at T = 298.15-313.15 K. The standard partial molar volumes. V-phi(0), partial molar volumes of transfer, Delta V-tr(phi)0 have been calculated using the measured density data. The cosphere overlap model was used to interpretate the partial molar volumes of transfer, Delta V-tr(phi)0. The calculated Hepler's constants, partial derivative V-2(phi)0/partial derivative T-2, indicate that the BPIE and ionic liquid have structure-breaking (chaotropic) tendency. Ion association constants. K-a, and limiting molar conductivities, Lambda(0), viscosity B-coefficients and molar refractions, R-D, have been used to confirm the conclusions obtained from the volumetric properties. The results were interpreted in terms of various solute-solvent interactions occuring in the investigated solutions and indicating an enhancement in the nonpolar-nonpolar interactions between the BPIE Schiff base and the ionic liquid with increasing BPIE concentration. (C) 2011 Elsevier B.V. All rights reserved.