Digital imaging and spectroscopic techniques, with high temporal and spatial resolution, were applied in order to study the low temperature combustion process. Injection and combustion phases were analysed by digital imaging. Mixing process, autoignition and pollutants formation were investigated by broad-band ultraviolet-visible extinction spectroscopy and flame emission measurements. Moreover, fuel distribution and oxidation were studied as well. Liquid fuel and vapour phase, injected around the top dead centre, were analysed. The liquid diesel fuel was observed by extinction measurements when the liquid jet reached the bowl rim and aromatic compounds due to fuel decomposition were identified. On the other side, the vapour fuel was detected about 2 degrees after the injection start and liquid fuel disappeared. Then, radicals and species were detected in the combustion chamber. They are interesting in order to study the chemical kinetics of low temperature combustion process. The chemiluminescence spectra of HCCI combustion appeared as well as several distinct peaks corresponding to the emission from HCO, HCHO, CH, and OH. In particular, this latter was clearly evident during the whole premixed combustion and dominated the process also after the end of the premixed phase of the heat release. Advancing the combustion, bright spots due to not homogeneous charge were detected. They were the source of the very little soot amount detected at the exhaust pipe. Finally, the injection pressure effect on the development of low temperature combustion was analysed. (C) 2010 Elsevier Ltd. All rights reserved.