Total hydrocarbon (THC) continuous emission monitors (CEMs), using flame ionization detectors (FIDs), are used to estimate total organic products of incomplete combustion (PICs) from combustion and incineration processes. A study has been conducted by the Air Pollution Prevention and Control Division of EPA's National Risk Management Research Laboratory to characterize, in greater detail, the FID response to a variety of representative combustion process emission products, including PICs. The objective of the study was to assess the accuracy and representativeness of THC FID measurements at low concentrations (< 10 ppm) by parametrically evaluating the FID response to various compounds under varied conditions. The FID responses of two different THC analyzers have been examined in detail with respect to calibration criteria and approach, the measurement of various hydrocarbon mixes, and the measurement of individual and combined combustion process emission components. The results presented indicate that a large number of variables contribute to the uncertainty of combustion THC measurements, particularly at low concentration levels.