The present paper reports results of density measurements for 1-C-n-3-methylimidazolium-based tetrafluoroborates and trifluoromethanesulfonates with n = 2 and 4 and with n = 2, 4, and 6, respectively. For the tetrafluoroborates, the measurements were performed at temperatures from (285 and 220) K, respectively, to approximately 357 K. For the trifluoromethanesulfonates, the measurements were conducted at temperatures from 353 K down to the melting temperature of the substance. Data at nominal pressures of 1 MPa and from (10 to 60) MPa with a 10 MPa step were obtained. An isochoric apparatus was used, making possible density measurements at temperatures below 273.15 K. The combined uncertainty at the 95 % confidence level in the resultant density data due to the measuring procedure is estimated to be 1 kg.m(-3), that is, about 0.1 % relative to the density value. Results of a quantitative analysis are reported of the effect of water and chloride anion impurities present in the sample on the density of the ionic liquids of interest. The sodium cation was studied experimentally as a candidate for the impurity causing an increase in ionic liquid density. At the same mass fraction, the sodium cation proves to be four times more effective than the chloride anion in changing the density of [HMIM][BF4].