The molar enthalpies of solution of ionic liquid (IL) [C(6)mim][Cl] (1-hexyl-3-methylimidazolium chloride) at various molalities in water were measured by a solution-reaction isoperibol calorimeter at 298.15 K. According to Pitzer's electrolyte solution theory, the molar standard enthalpy of solution of [C(6)mim][Cl], Delta H-sol(m)o and Pitzer's parameters, beta((o)L)(MX), beta((I)L)(MX), and C-MX(phi L), were obtained. In terms of the experimental data in literature, the mean contribution per methylene (-CH2-) group to the standard molar enthalpies of solution of [C(x)mim][Cl] is discussed.