New experimental data on the density of two 1-alkyl-3-methylimidazolium based dicyanamides ([C-n-MIM] [N(CN)(2)], n = 2 and 4) are reported, obtained with a constant volume apparatus, which makes possible density measurements at subzero temperatures. The measurements were conducted at temperatures from 350 K down to the melting temperature of the substance and at nominal pressures of 1 MPa and from (10 to 60) MPa with a 10 MPa step. The combined uncertainty at the 0.95 confidence level of the resultant density data is estimated to be less than 1 kg.m(-3), that is, 0.1 % relative to the density value. A group contribution model of the p-rho-T relation based on a large amount of data by different authors for 1-C-n-3-methylirnidazolium-based ionic liquids with n = 2, 4, and 6 and with the [BF4], [CF3SO3], and [N(CN)(2)] anions describes the dependence of density, isobaric expansivity, and isothermal compressibility on temperature, pressure, the length of the alkyl side chain of the cation, and the anion. This makes it possible to obtain the most reliable values of the density and to assess which sets of data can be considered as more certain than others.