We describe a novel method of obtaining the fuel concentration at various points along the longitudinal axis of a spark ignition engine. This allows us to measure the effect of various engine and fuel parameters on charge stratification in the combustion chamber. The technique is based on sampling the infrared (IR) absorption spectrum through the transparent sapphire engine cylinder. The absorbance is measured at four points down the chamber axis using four IR launching fibers, each of which is matched to a collection fiber. The spectra are simultaneously dispersed and stacked on a diode array detector. We subtract the background from the absorption peak to obtain the absorbance which is shown to be directly proportional to fuel concentration. We find that the axial charge stratification and its evolution during the induction and compression strokes is a sensitive function of injection timing and swirl valve setting. The technique is also applicable to gas phase reactors and any system where spatially homogeneous mixing of reactants is required. ©1997 American Institute of Physics.