Densities as a function of temperature (283.15-333.15 K) and pressure (0.1-60 MPa) for the isomeric ionic liquids 1-butyl-2-methylpyridinium tetrafluoroborate, 1-butyl-3-methylpyridinium tetrafluoroborate, and 1-butyl-4-methylpyridinium tetrafluoroborate have been determined. Experimental data have been correlated using two different equations for evaluating their suitability to describe the volumetric behaviour of these materials. In addition, derived properties such as the coefficient of thermal expansion, the isothermal compressibility and the internal pressure have been calculated from densities making use of these two expressions. A detailed analysis of data in terms of the structural characteristics has been carried out. Thus, a comparison of the results for the fluids studied in this work has been used to provide some insight to the influence of the substituents position on their behaviour. (C) 2011 Elsevier B.V. All rights reserved.