New experimental data on the density of 1-butyl-3-methylimidazolium tetrafluoroborate are reported, obtained with a constant volume apparatus described here in detail. The measurements were conducted along eight isochores at temperatures from (354 to 240) K and at nominal pressures of 1 MPa and from (10 to 60) MPa with a 10 MPa step. The combined uncertainty at the 95% confidence level of a density measurement is estimated to be less than 1 kg.m(-3), i.e., 0.1 %. A Tait-type equation was developed describing the rho(T, p) dependence. The equation is fitted to the data of the present work and to critically assessed data of other authors compiled from the literature. Values of the coefficient of isothermal compressibility and of isobaric thermal expansion for 1-buty1-3-methylimidazolium tetrafluoroborate were calculated from the correlation equation and compared with experimental data from the literature. An unusual decreasing dependence of the isobaric thermal expansion coefficient on temperature, observed by other authors, was confirmed.