In an effort to obtain a better under-standing of the mechanisms leading to the presence of unburnt hydrocarbons (uHC) in the exhaust gases from automotive gasoline engines, a fast-response, flame ionization detector has been deployed in the exhaust port of a firing engine. The detector used has a response time of approximately 1 ms and so was able to follow changes in uHC concentration levels during the exhaust stroke. After checking that the signal from the detector was not corrupted by pressure or temperature excursions in the exhaust port, the instrument was used to record uHC emissions signatures at various stations in the exhaust port of a single cylinder engine. Measurements were made at intervals of 2 degrees of crankangle movement (CA) during steady-state engine operation at 2000 r/min, 2 bar bmep. The shape of the uHC versus CA signature was found to change as the probe sampling head was moved downstream from the exhaust valve. The reasons for these changes were then explored. Parameters such as mass flow, pressure and velocity were predicted using an engine simulation program. These predictions were then used in a plug flow model to predict the movement of pockets of gas leaving the cylinder. When this plug flow model was calibrated with a measured uHC/signature at one location it gave good predictions of signatures at other locations. The variations with location of uHC signature were thus demonstrated to be largely due to the Intermittent motion of gas within the exhaust system.