Abstract.Spectrally resolved visible and ultraviolet emissions are investigated as a basis for wide-range, individual-cycle measurement of the local fuel concentration in spark-ignition engines. The 388-nm CN emission intensity, normalized by the spark-discharge energy during the observation interval (typically 150 μs at the start of the glow discharge), is found to be the most useful measure of fuel concentration when data are required over a wide range. Calibration data for homogeneous propane–air and isooctane–air mixtures over a wide range of cylinder gas conditions at the time of ignition collapse to a single curve when the fuel concentration is expressed in terms of the number density of carbon atoms. The carbon number densities measured in this study correspond to fuel–air equivalence-ratios in the range 0–3 at 95% throttle conditions. Random and systematic errors are 10% or less. Applied to an engine in which liquid fuel is injected directly into the cylinder, the technique reveals substantial cyclic fluctuations in the fuel concentration at the spark gap for early fuel injection (intended to produce a homogeneous fuel–air mixture in the combustion chamber) and large fuel-concentration fluctuations for late fuel injection (which produces a highly stratified mixture). The results also show that for stratified operation with a fixed fuel-injection timing, a spark timing that is later than optimum leads to incomplete combustion in many cycles due to fuel–air ratios that are too lean for good ignition and rapid flame development.