A hybrid of isoperibol and phase-change bomb calorimeters with gallium as melting body was developed. This calorimeter allows one to raise the temperature of the reagents in the bomb to about 1100 K during (45 to 50) min. The procedure of calibration of the calorimetric system is described; the molar enthalpy of melting of gallium which was measured in the calibration process was found : (5584.7 +/- 0.5) J . mol(-1). The molar enthalpy of the reaction : Na2C2O4 + 0.5O(2) = Na2CO3 + CO2, was measured for testing the new system. The molar enthalpy of formation of Na2C2O4 calculated : -(1322.3 +/- 1.0) kJ . mol(-1), agrees with the value : - (1322.3 +/- 1.4) kJ . mol(-1), obtained by solution calorimetry.