The molar enthalpies of solution, Delta H-sol(m), for ionic liquid [C(5)mim][OAc] with different molalities were measured by the solution-reaction isoperibol calorimeter in the temperature range from 288.15 to 308.15 K with an interval of 5 K. According to Archer's method, the standard molar enthalpies of solution, Delta H-sol(m)theta, en thalpy, and the apparent relative molar enalpy, L-Phi, for [C(5)mim][OAc] were determined. In terms of Glasser's theory of lattice energy, the hydration enthalpy of the cation, Delta H+ ([C(5)mim](+)) = -178 kJ. mol(-1) was obtained at 298.15 K. Compared with hydration enthalpy of [C(2)mim](+), Delta H+, ([C(2)mim](+)) = -233 kJ.mol(-1), the hydration of [C(5)mim](+) is weaker than [C(2)mim](+). The plot of Delta H-sol(m)theta, vs (T - 298.15) K is a good straight line and its slope is the standard molar heat capacity of solution, Delta C-p,m(theta) = 231 J.K-1.mol(-1), so that the specific heat capacity of solution, Delta C-p(theta) = 1.09 J. g(-1).K-1, was obtained for IL [C(5)mim][OAc].