Experimental air-liquid interfacial tension data and density data are presented for three 1-C-n-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphates (FAP), [CnMIM][(C2F5)(3)PF3], with n = 2, 4, and 6, measured at atmospheric pressure in the temperature range from 267 K to 360 K using the Kr} uss K100MK2 tensiometer. The accuracy of the surface tension measurements was checked by employing the Wilhelmy plate and the du Nouy ring methods in parallel. The combined standard uncertainty associated with the Wilhelmy plate method is estimated to be +/- 0.1 mN . m (1). The density data were obtained using buoyancy method with an estimated standard uncertainty less then +/- 0.4 kg . m (3) (3 . 10 (4)rho). The chloride anions decrease the density of the tris(pentafluoroethyl) trifluorophosphates of interest up to six times more effectively than they decrease the density of the imidazolium based tetrafluoroborates. A QSPR analysis of the surface tension of imidazolium based ionic liquids with BF4, TFA, DCA, FAP, NTf2, and PF6 anions indicates, that the FAP ionic liquids fit well into the analyzed group of imidazolium based ionic liquids while those having hexafluorophosphate anion show anomalously high deviations of the experimental surface tension from the values predicted by the QSPR model. (C) 2012 Elsevier Ltd. All rights reserved.