An investigation of the effects of contour conditions and fuel properties on the auto-ignition and combustion process under HCCI conditions is presented in this study. A parametric variation of initial temperature, intake pressure, compression ratio, oxygen concentration and equivalence ratio has been carried out for Primary Reference Fuels in a Rapid Compression Expansion Machine while applying spectroscopy. The results have also been contrasted with natural chemiluminescence measurements. Additionally, the experiments have been simulated in CHEMKIN and the results derived from the optical techniques have been compared with the results from the chemical kinetics of the process, validating the chemical kinetic mechanism and an additional sub-model of excited OH*. Two different scenarios can be seen according to the results from the spectrograph. For very lean or very low-temperature combustions no peak of OH* is seen at 310 nm of wavelength, proving that the luminosity came from the CO continuum rather than from the OH*. However, for more intense combustions (richer equivalence ratios, higher temperatures or lower EGR rates) spectrography shows a clear peak of OH* that has much longer time of life than the corresponding to the CO continuum. The main chemical reaction that causes this two scenarios has been identified as H + HO2 double right arrow 2OH. The increase of relevance of this reaction at high combustion temperatures causes a higher OH* accumulation, which leads to a brighter OH* emission. Finally, for low temperature combustions the CO continuum out-shines the OH* radiation so the light emitted by this radical cannot be detected by means of natural chemiluminescence. (C) 2016 Elsevier Ltd. All rights reserved.