Local, crank angle resolved fuel density variations were detected with a fiber optical sensor system in the vicinity of the spark position in a cylinder of a four-stroke SI production engine. The infrared absorption sensor detects the attenuation of infrared radiation in the 3.4 μm wavelength region due to the infrared vibrational-rotational absorption band of hydrocarbons (HC). The absorption path was integrated in a modified spark plug and a tungsten halide lamp was used as an infrared light source. All investigations were carried out on a four-stroke spark ignition engine with fuel injection into the intake manifold. The engine operated with common gasoline (Euro Super) at different preset overall air/fuel-ratios. Fuel concentration variations and the mixing process before the combustion were investigated for single engine cycles. In order to derive air/fuel-ratios from the measured HC-concentrations a special approximation method was used.