An infrared fiber optic instrumented spark plug probe has been used to measure the fuel concentration in the vicinity of the spark gap in a port injected gasoline fueled SI engine. The probe measured the fuel concentration spatially averaged over a distance of 6.3 mm near the spark plug for consecutive firing cycles. The crank angle resolution of the measurements was 2.5 degrees, for a temporal resolution of between 0.9 and 0.3 ms depending on the engine speed. Quantitative measurements of the fuel concentration in the pre-ignition regions of the engine cycle were obtained. Qualitative results are reported for unburned hydrocarbons in the post-combustion regions. The measurements were made in a single cylinder research engine over a range of speed, load, and stoichiometric conditions. Strong mixture inhomogeneities were measured during the intake stroke and the inhomogeneities decreased through the compression stroke. Increasing engine speed resulted in higher levels of inhomogeneities and cycle to cycle variations in the fuel distribution, but the overall mixing process scaled with the engine speed. Changes in overall equivalence ratio and manifold air pressure did not dramatically affect the mixing process.