The accuracy claimed for calorimetric measurements should be given at a certain confidence level. This paper describes how the standard uncertainty of isobaric enthalpy increment measurements from a new cryogenic flow calorimeter is derived. The new calorimeter has several features that reduce the heat leakage better than previous designs. The temperature and pressure ranges covered by-the calorimeter are 133-335 K and 0.17-14 MPa. The enthalpy increment measurements have an average standard uncertainty of 0.10 kJ kg(-1) or 0.25% of the enthalpy increment.