Transient, precombustion fuel/air ratios at the spark plug electrode gap in a DISC engine have been measured using cw laser Raman spectroscopy and have been correlated with mixture ignitability measurements. From these measurements, it was found that with decreasing quantities of injected fuel there was a general trend towards decreased peak equivalence ratio, a delay in fuel arrival at the spark electrode gap, and a narrowing of the crank angle duration over which mixture ignition was possible. Lean inflammability limits determined from the ignition study suggested a dual-ignition mechanism which depended on the rate of change of equivalence ratio with crank angle. Flow visualization of the stratified charge combustion revealed substantial differences in mixture burn characteristics and combustion duration as the ignition crank angle was varied. Quenching of the burning mixture was observed and its occurrence was correlated with the Raman measurements.