In this paper, a densimeter based on vibrating tube principle is used to determine experimentally the density of 1-butyl-2,3-dimethylimidazolium tris(pentafluoroethyl) trifluorophosphate and 1-butyl-2, 3-dimethylimidazolium bis(trifluoromethylsulfonyl) imide at temperatures between (278.15 and 398.15) K and at pressures up to 120 MPa. The apparatus was calibrated by using water, vacuum and bromobenzene. The Tammann-Tait equation of state was used to correlate (p, T, rho) results with standard deviations around 2 . 10 (4) g . cm (3). Other volumetric properties, such as isothermal compressibility and isobaric thermal expansivity, were obtained fromthis equation. For each ionic liquid, the alpha(p) isotherms present a crossing point within the experimental pressure range. Besides, the effect that the C2-methylation in the imidazolium cation provokes on density values is analyzed. The prediction ability of the group contribution methods of Gardas and Coutinho and Jacquemin et al. were tested with the experimental densities. (C) 2014 Elsevier Ltd. All rights reserved.