The molar enthalpies of solution of ionic liquid 1-methyl-3-butylimidazolium tetrafluoroborate (BMIBF4) in water at various molalities were determined by calorimetry at 298.15 K. Pitzer's electrolyte solution theory was used to obtain the molar solution enthalpy of BMIBF4 at infinite dilution, Delta(s)H(m)(0) = 26.82 +/-0.26 kJ/mol. (C) 2005 Elsevier B.V. All rights reserved.