Experimental air-liquid interfacial tension data and density data are presented for two ionic liquids with the 1-C(n)-3-methylimidazolium cation, n = 2 and 4, and the dicyanamide anion. The measurements were performed at atmospheric pressure in the temperature range from (278 to 356) K using the Kruss K100MK2 tensiometer. The accuracy of the surface tension measurements was confirmed by employing the Wilhelmy plate and the du Nouy ring methods in parallel. The combined expanded uncertainty at 0.95 confidence level associated with the Wilhelmy plate method is estimated to be 0.1 mN.m(-1). The density data were obtained using a buoyancy method with an estimated expanded uncertainty at 0.95 confidence level of 3.10(-4)rho. An analysis of the experimental density data, using a group contribution model, performed for eight ionic liquids with the 1-C(n)-3-methylimidazolium cation (n = 2, 4, 6) and tetrafluoroborate and trifluoromethanesulfonate and the dicyanamide anion indicates that the uncertainty arising from impurities present in the sample is dominant. The upper-bound estimate for the overall expanded uncertainty (k = 2) in the density measurements including the uncertainty stemming from impurities in the sample is equal to +/- 0.002 rho. The model is capable of providing predictions for pure imidazolium-based ionic liquids with [BF(4)], [CF(3)SO(3)], and [N(CN)(2)] anions.