A newly developed measurement technique was employed to investigate flame propagation before and during knocking combustion. Two intensified CCD cameras were used in sequence to record the natural flame light during knocking combustion via a fused silica window, which was fitted to the cylinder head of a one-cylinder four-stroke SI-engine. This arrangement enabled acquisition of two images per cycle. While the first camera was triggered by the rapid rise of the cylinder pressure at the beginning of engine knock, the second camera was activated at a prescribed time of delay.Due to the high sensitivity of the intensified CCD cameras, extremly short exposure times of 100 to 250 ns proved to be sufficient. Consequently, acquisition rates of 200 kHz and more - which are substantially higher than those of conventional natural light photography - could be realized. Acquisition rates of 100 kHz (Δtacq. = 10 μs) seem adequate to trace the rapid propagation of reaction fronts immediately after the onset of knock. By triggering the first camera through the “knock generated pressure” pulse, we evaded the problems associated with the random occurrence of engine knock.Extensive measurements for two different combustion chamber configurations were made at different engine speeds. The octane rating was varied by mixing the primary reference fuels i-octane and n-heptane in different proportions.The experiments clearly show that engine knock was locally induced by spontaneous ignition in the end-gas region rather than acceleration of the primary flame. The location and mode of the ensuing “secondary” propagation wave varied over a wide range, depending among other factors on octane rating and thermodynamic conditions.Flame velocities measured directly after the onset of knock indicate that the propagation mode of the reaction wave is somewhere between deflagration and developing detonation.