Physical properties like density, viscosity, surface tension and the thermal stability of a series of pyridinium ionic liquids (ILs) with different cationic structures: 1-ethylpyridinium [C(2)py], 1-butylpyridinium [C(4)py], 1-butyl-2-methylpyridinium [C-4-2-C(1)py], 1-butyl-3-methylpyridinium [C-4-3-C(1)py], 1-butyl-4-methylpyridinium [C-4-4-C(1)py], 1-hexylpyridinium [C(6)py] combined with an anion bis(trifluoromethylsulfonyl) imide [NTf2], and 1-butyl-3-methylpyridinium trifluoromethanesulfonate [C-4-3-C(1)py][OTf], 1-butyl-3-methylpyridinium tetrafluoroborate [C-4-3-C(1)py][BF4], and 1-butyl-3-methylpyridinium tris(pentafluoroethyl) trifluorophosphate [C-4-3-C(1)py][FAP] were measured over a wide temperature range. Furthermore the Arrhenius equation and Vogel-Fulcher-Tammann equation have been used to correlate the variation of viscosity of studied ILs with the temperature. Moreover entropy and enthalpy of surface formation per unit surface area of pyridinium ILs have been calculated. The thermal stability of all pyridinium ILs was also investigated at heating rate of 15 K.min (1), in the temperatures range 300 to 1000 K using thermogravimetric analysis instruments (TGA). (C) 2012 Elsevier Ltd. All rights reserved.