The cycle-resolved fuel concentration near the spark plug in a firing SI engine has been measured using an infrared fiber optic instrumented spark plug probe. The probe can measure in-cylinder concentrations of hydrocarbons in the pre-combustion regions of the engine cycle and give qualitative results for unburned hydrocarbons in the post-combustion regions. The device consists of a spark plug body that has been modified to accept a pair of sapphire optical fibers in addition to a spark electrode. Radiation from an infrared source is coupled into one fiber and reflected from a minor on the spark plug ground electrode to the other fiber which carries the signal to a detector and data acquisition system. The probe measures the attenuation of the infrared radiation transmitted through a region in the vicinity of the spark gap. The attenuation results from the absorption of radiation by the fuel.The measurements were made in a CFR engine at 600 rpm using propane fuel. The probe measured the fuel concentration spatially averaged over a distance of 4.5 mm near the spark plug for consecutive firing cycles. The temporal resolution of the measurements was 2.4 crank angle degrees. Three air/fuel equivalence ratios were examined for a single load condition. The differences between motoring and firing operation were also examined. Strong mixture inhomogeneities were measured during the intake stroke. The mixture in the vicinity of the spark plug became relatively homogeneous by the middle of the compression stroke. For near stoichiometric combustion it appears that unburned hydrocarbons out-gassing from the crevices reached the spark plug beginning about 70° aTDC.