A fast-response FID has been used to study the concentration of hydrocarbon material at four different locations in a firing SI engine. These were: on the flat surface of the cylinder head, at the exhaust valve seat crevice, just downstream of the exhaust seat in the exhaust port and 20mm downstream from the valve stem in the exhaust manifold. A close-fitting sleeve arrangement enabled the sample tube to be positioned accurately flush with the head face and also to be slid away from the wall into the bulk gases whilst maintaining a gas-tight seal. In this way, wall effects could be noted by moving the probe position without stopping the engine and directly comparing with hydrocarbon levels in the bulk gas. Using propane in a fully-warmed up engine, results showed the presence of HCs residing in a 2mm layer adjacent to the wall after EVO and during the exhaust stroke. These could be detected flowing over the valve seat after EVO and were also observed at the manifold location. This indicated that quench and crevice HCs may be transported by engine gas dynamics within a layer adjacent to the head surface into the vicinity of the sampling points avoiding extensive oxidation in the bulk cylinder gases.