Pressure, density, temperature (p, p. T) data of 1-butyl-3-methylpyridinium tetrafluoroborate [C4mpyr][BF(4)] at T = (283.15 to 393.15) K and pressures up to p = 100 MPa are reported with an estimated experimental relative combined standard uncertainty of Delta rho/rho = +/-(0.01 to 0.08)% in density. The measurements were carried out with a newly constructed Anton-Paar DMA HPM vibration-tube densimeter. The system was calibrated using double-distilled water, methanol, toluene and aqueous NaCl solutions. An empirical equation of state for fitting of the (p, rho, T) data of [C4mpyr][BE(4)] has been developed as a function of pressure and temperature to calculate the thermal properties of the ionic liquid (IL), such as isothermal compressibility, isobaric thermal expansibility, differences in isobaric and isochoric heat capacities, thermal pressure coefficient and internal pressure. Internal pressure and the temperature coefficient of internal pressure data were used to make conclusions on the molecular characteristics of the IL. (C) 2011 Elsevier Ltd. All rights reserved.