The enthalpy or calorimetric probe is a water‐cooled stagnation/sampling probe for studying the temperature, velocity, and composition of hot‐gas flow fields. In order to derive the thermodynamic properties of complex flow fields such as plasma arc jets or high‐velocity oxygen fuel jets, the specie concentration must be known accurately. To this end a differentially pumped quadrapole mass spectrometer has been integrated with a fully automated enthalpy probe system. An inexpensive modular probe is described along with the system and its theory of operation. Calibration and error are also discussed. Typical results are presented for the system operating in an argon/helium plasma arc jet in atmospheric pressure air. The maximum temperature measured is 13434 K on the center line of the jet, 5 mm from the exit, with a corresponding velocity of 1295 m/s. The utility in integrating the mass spectrometer to the enthalpy probe system is not only an accurate measurement of the gas mixture components for obtaining the correct property information, but also valuable information can be obtained about demixing diffusion and chemical reaction taking place in the plasma plume. The relative amount of argon to helium is shown to deviate from the nominal mixture by as much as 40% at the center of the plume.