The environmental effects of particulate matter (PM) emissions from vehicles are an increasing concern to those concerned with air quality. A variety of technologies have been developed to measure exhaust particulates. The automotive industry generally uses the gravimetric method to quantify particulate emissions. This method uses a combination of a dilution tunnel and filter to collect PM from the diluted sample gas. The collected PM is later weighed on a microbalance. Because this technique is a batch measurement, it is not possible to determine at what point of an emissions test drive cycle the soot, soluble organic fraction (SOF) and total PM are emitted. A more accurate characterization of PM emissions will require real-time PM measurement under transient test conditions.A fast-response heated flame-ionization detector (fast-FID) has previously been shown to detect hydrocarbon-free soot particles by accumulating the area of spike signals that are observed when the fast-FID is fed with soot particles. This paper explains a new technique for continuously measuring soot and SOF separately, using the differential FID method. In addition to the basic principles, the authors explain the signal processing to measure soot and SOF from FID signals. In addition, test results from measurements taken from an actual powertrain are discussed.