The enthalpy of the 1-butyl-3-methylimidazolium bromide [C(4)mim]Br ionic liquid synthesis reaction 1-methylimidazole (liq) + 1-bromobutane (liq) -> [C(4)mim]Br (liq) was determined in a homemade small-volume isoperibol calorimeter to be Delta H-r degrees(298) = -87.7 +/- 1.6 kJ.mol(-1). The activation energy for this reaction in a homogeneous system E-A = 73 +/- 4 kJ.mol(-1) was found from the results of calorimetric measurements. The formation enthalpies for the crystalline and liquid [C(4)mim]Br were determined from the calorimetric data: Delta H-f degrees(298)(cr) = -178 +/- 5 kJ.mol(-1) and Delta H-f degrees(298)(liq) = -158 +/- 5 kJ.mol(-1). The ideal-gas formation enthalpy of this compound A(f)H degrees(298)(g) = 16 +/- 7 kJ.mol(-1) was calculated using the methods of quantum chemistry and statistical thermodynamics. The vaporization enthalpy of [C(4)mim]Br, Delta H-vap degrees(298) = 174 +/- 9 kJ.mol(-1), was estimated from the experimental and calculated formation enthalpies. It was demonstrated that vapor pressure of this ionic liquid cannot be experimentally determined.