Density and surface tension of the air- and water-stable hydrophobic ionic liquids N-alkylpyridinium bis(trifluoromethylsulfonyl)imide ([CnPY][NTf2], n = 2, 4, 5) were measured in the temperature range T = (283.15 to 338.15) K. The melting temperatures of the samples were determined by differential scanning calorimetry (DSC). Decomposition temperatures are higher than 600 K as determined by thermogravimetric analysis (TG).