The binding of 1-alkyl-3-methylimidazolium ions (C(n)mim(+)) to p-sulfonato-substituted calix[4]arene (SCX4) and calix[6]arene (SCX6) was studied by isothermal titration calorimetry in aqueous solution at 298 K. The complexation had 1:1 stoichiometry and was enthalpy driven for all cations. The equilibrium constants of inclusion were determined at pH 6.5 and compared to those previously reported at pH 2. The driving force of encapsulation showed smaller pH dependence for SCX4 than for SCX6. The binding enthalpy changed only slightly when the 1-alkyl moiety of the imidazolium ring was lengthened and did not depend on the size of the calixarene macrocycle. The complexation with SCX4 caused smaller entropy loss and steeper linear enthalpy-entropy correlation. (C) 2011 Elsevier B.V. All rights reserved.