A novel adiabatic calorimeter was constructed for simultaneous measurement of enthalpy and volume under high pressure. The calorimeter works under constant pressure up to 100 MPa and in the temperature range 80–380 K. The sample is pressurized hydrostatically by using a liquid pressure transmitting medium. The sample volume is measured with a new type of dilatometer using bellows installed in the cell. The dilatometer has two resolution modes (high and low) with different scanning range. As the calibration and test experiments, the heat capacities and volumes of toluene and water were measured at 0.1 and 50.1 MPa in the temperature range between 275 and 380 K. In the low resolution mode, the accuracies of heat capacity and volume were±0.2&percent;and 20 ppm of total, respectively. The test of the volume measurement in the high resolution mode showed that the resolution was±0.1 ppmof total sample volume, which is ∼ ten times better than that of the previous best dilatometer. To demonstrate the performance of the present apparatus, the enthalpy and volume changes due to the structural relaxation of glassy polystyrene were measured simultaneously. ©1998 American Institute of Physics.