Density and viscosity data for six pyridinium-based ionic liquids combined with the bis[(trifluoromethyl)-sulfonyl]amide anion were measured at atmospheric pressure in the (278 to 363) K temperature range. The fundamental aim of this work is to study the effect or the structure of the pyridinium-based cation, namely, its alkyl chain length and structural isomers, and the position of the second alkyl substitution on the measured properties. Albeit many studies exist on the physical properties of ionic liquids in what concerns the nature of the anion and the length of the cation alkyl side chain, the effect of structural and positional isomerism on those properties is much less known. In addition-and since small amounts of water influence the phase equilibrium and thermophysical properties of ionic liquids-water-saturated ionic liquid samples were also studied in the (298 to 363) K temperature range. The Vogel-Tammann-Fulcher (VTF) method was applied to describe the viscosity data, and novel group contribution parameters are proposed for the ionic liquid cations presented here, thus broadening its applicability.