Simultaneous measurements of planar laser-induced fluorescence of formaldehyde and high-speed chemiluminescence are performed in the cubic combustion vessel of a rapid compression machine. From both early and late stages of the process, formaldehyde appears as the “negative” of flame chemiluminescence. Apparition of formaldehyde also coincides with the first rise in pressure identified as the cool flame. Self-ignition then Spark Induced Compression Ignition (SICI) regimes are investigated, bringing insights into the understanding of both phenomenologies. From a quantitative point of view, an analysis of apparent flame velocities of the reaction zones is presented, focusing on the early stage of the combustion process. This simultaneous diagnostic finally leads to a better understanding in the propagative modes, namely deflagration and auto-ignition, implied in such combustion processes where complex interactions between temperature heterogeneities, aerodynamics and chemistry of the reactive processes take place.