The accuracy of the heat capacity of a liquid by the calorimetric method depends on the corrective term due to the vapor phase above the liquid. In order to overcome this difficulty, a special calorimetric vessel has been designed. The main feature is a tube welded to the experimental vessel. Its filling is done through the tube by means of a syringe, until there is liquid in the tube. The top of the vessel is machined so that there is no vapor or bubble retained in the experimental vessel. When the liquid is heated the liquid expands freely in the tube, but the volume of liquid in the vessel, located in the direction zone of the calorimeter remains constant. The determination of the heat capacity of this corresponding volume is achieved using the step-heating mode.