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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.