Three major problems encountered in measuring concentration and mass emissions of hydrocarbons from engines using flame ionization detectors (FIDs) were studied in detail. The magnitude of possible correlation discrepancies for analyses run on various commercial instruments as a function of operating conditions and exhaust type was estimated from measured relative molar response values. The larger analyses values occasionally observed with heated analyzers may result from increased relative molar responses to the various individual hydrocarbons rather than decreased line absorption effects. The magnitude of synergism, important for real-time automotive testing, was measured on various FIDs at different conditions. A simple modification to minimize the effect is recommended for the majority of analyzers that exhibit this feature. An extensive study of instrument response times was completed for a variety of sampling lines. The relative importance of factors, such as the analyzer operating temperature, sample line temperature, instrument sample pressure, bypass flow rate, sample line length, diameter, and material, was established. Optimum response was realized with a heated FID using heated /sup 1///sub 4/-in outside diameter stainless steel or Teflon lines and a high bypass sample flow rate.