The thermal mass flow controller (MFC) is the upstream gas metering device chosen for a majority of vacuum processes. The purpose of this paper is to identify the important physical phenomena and key parameters which govern the performance of MFCs, but which have been previously unmentioned in the literature. Detailed analytical, simulation, and experimental results define the roles of the various heat transfer mechanisms which characterize the behavior of an MFC. The effect of the gas species is of particular, practical concern in view of the wide variety of gases used in processes and the need for higher levels of measurement reliability. The research shows that for some gases the present use of a gas correction factor should be replaced by a gas correction function.