Particulate number (PN) standards in the current ‘Euro 6’ European emissions standards pose a challenge for engine designers and calibrators during the warm-up phases of cold direct injection spark ignition (DISI) engines. To achieve catalyst light-off in the shortest time, engine strategies are often employed which inherently use more fuel to attain higher exhaust temperatures. This can lead to the generation of locally fuel-rich regions within the combustion chamber and the emission of particulates.This investigation analyses the combustion structures during the transient start-up phase of an optical DISI engine. High-speed, colour 9 kHz imaging was used to investigate five important operating points of an engine start-up strategy whilst simultaneously recording in-cylinder pressure.Results show a striking range of flame structures occurring at different times in the start-up cycle with significant differences in flame propagation rates, flame front structures and local air-fuel ratios (AFRs). High-luminosity regions, occurring due to observed soot incandescence, identify distinct regions of potential soot generation. The processed pressure data shows a clear link between engine performance and flame growth speed and structures.