A specific calibration procedure that allows the accurate determination of densities of room temperature ionic liquids, RTILs, as a function of temperature and pressure using vibrating tube densimeters is presented. This methodology overcomes the problems of common calibration methods when they are used to determine the densities of high density and high viscosity fluids such as RTILs. The methodology is applied for the precise density determination of RTILs 1-ethyl-3-methylimidazolium tetrafluoroborate [Emim][BF4], 1-butyl-3-methylimidazolium tetrafluoroborate [Hmim][BF4], and 1-octyl-3-methylimidazolium tetrafluoroborate [Hmim][BF4], and 1-octyl-3-methylimidazolium tetrafluoroborate [Omim][BF4] in the temperature and pressure intervals (283.15 to 323.15) K and (0.1 to 60) MPa, respectively. The viscosities of these Substances, needed for the estimation of the viscosity-induced errors, were estimated at the same conditions from the experimental measurements in the intervals (283.15 to 323.15) K and (0.1 to 14) MPa and from a specific extrapolation procedure. The uncertainty in the density measurements was estimated in +/- 0.30 kg . m(-3) which is all excellent value in the working intervals. The results of these RTlLs have demonstrated that viscosity-induced errors are relevant and they must be taken into account for a precise density determination. Finally, ail alternative tool for a simpler application of this procedure is presented. (C) 2009 Published by Elsevier Ltd.