In this study, optical measurements were made of the combustion chamber gas during operation of a Homogeneous Charge Compression Ignition (HCCI) engine in order to obtain a better understanding of the ignition and combustion characteristics. The principal issues of HCCI engines are to control the ignition timing and to optimize the combustion state following ignition. Autoignition in HCCI engines is strongly influenced by the complex low-temperature oxidation reaction process, alternatively referred to as the cool flame reaction or negative temperature coefficient (NTC) region. Accordingly, a good understanding of this low-temperature oxidation reaction process is indispensable to ignition timing control.In the experiments, spectroscopic measurement methods were applied to investigate the reaction behavior in the process leading to autoignition. Two types of spectroscopic measurements were obtained: measurement of spontaneous light emission from the flame in the combustion chamber and measurement of light absorption. The former measurement facilitated detection of light emission from chemical species produced by the reactions; the latter method facilitated measurement of the production and consumption behavior of the chemical species even when no light emission occurred. Following ignition, combustion (hot flame) proceeds rapidly under a high load, giving rise to knocking and the production of nitrogen oxides (NOx) by the high-temperature gas. Engine operation under a low load is restricted by incomplete combustion and misfiring due to the lower temperature. Therefore, the characteristics of the hot flame following ignition were investigated by measuring the light emission spectrum of the flame and by visualizing the conditions in the cylinder.Using these methods, the light emission spectrum was measured during operation of the experimental HCCI engine. The results made clear the light emission spectrum of the hot flame following ignition was made clear. The main emission was found to be a continuous spectrum from 300 nm to 500 nm, presumably attributable to the recombination reaction of carbon monoxide and oxygen atoms and known as CO-O glow. The results also revealed that the time of light emission in a wavelength region of approximately 300-500 nm, corresponding to the CO-O glow, coincided well with the time of heat release. This implies that spectroscopic measurement of light emission in the wavelength region corresponding to the CO-O glow can facilitate detection of local heat release in the cylinder.In-cylinder visualization was used to obtain two-dimensional flame images across the entire cylinder bore area in order to investigate the relationship between the local state of hot flame development and the rapidity of combustion. The results revealed that the time for the occurrence of autoignition showed little local difference under conditions conducive to rapid combustion; autoignition occurred almost simultaneously throughout the unburned region and the mixture burned all at once.