Despite many previous important contributions to the characterization of the liquid-liquid phase behavior of ionic liquids (ILs) plus water systems, a gap still exists as far as the effect of isomers (of ILs) is concerned Therefore in this work, a comprehensive study of the liquid-liquid equilibria between water and isomeric pyridinium-based ionic liquids has been performed Atmospheric pressure mutual solubilities between water and pyridinium-based ionic liquids combined with the common anion bis[(trifluoromethyl)sulfonyl]imide were experimentally determined between (288 15 and 318 15) K The main goal of this work is to study the isomeric effects on the pyridinium-based cation namely, the structural and positional isomerism, as well as the alkyl side chain length To the best of our knowledge the influence of both structural and positional isomerism on the liquid-liquid behavior in ionic-liquid-water-containing systems is an unexplored field and is here assessed for the first time Moreover, from the experimental solubility data several infinite dilution molar thermodynamic functions of solution, namely the Gibbs energy, the enthalpy, and the entropy were estimated and discussed In addition, aiming at gathering a broader picture of the underlying thermodynamic solvation phenomenon molecular dynamics simulations were also carried out for the same experimental systems