In this communication, we present an interesting reversal in the enthalpic behavior from endothermic to exothermic on increasing the carbon chain length attached to the imidazolium ring in the imidazolium-based ionic liquids. A systematic investigation is presented on the excess partial molar enthalpy, HILE directly measured using isothermal titration calorimeter at 298.15K for the interactions operating between cation and anion of four imidazolium ionic liquids with aqueous and various non-aqueous solvents. This reversal behavior is analyzed in terms of hydrophobic and solvation effects.