Within the last few years, the understanding of the origins of engine knock has advanced considerably. Numerical experiments have shown that inhomogeneities in temperature distribution of premixed gases of only 10 K, or even less, can lead to the onset of autoignition. This is called ignition by exothermic centres (ETCs) or, more colloquially, by “hot spot”. Still, there is no experimental technique available to detect such small temperature fluctuations directly. In an earlier paper, the existence of hot spots was verified by application of two-dimensional laser-induced fluorescence (2D-LIF) using formaldehyde as tracer. In the two-stage ignition of hydrocarbons, formaldehyde is formed early and then is decomposed very rapidly after ignition. The core of the experimental set-up consists of a modified, single-cylinder, two-stroke engine with full optical access to the combustion chamber from above. The engine is run with primary reference fuel (PRF), a mixture of n-heptane and iso-octane. Two exciner lasers, coupled into one dye laser, make the double-pulse excitation of formaldehyde possible. The fluorescence signals are detected with two intensified, charge-coupled device (ICCD) cameras, amplified, and stored in a PC. Under different knocking conditions, pairs of two-dimensional LIF pictures, separated by 50 μs and less, and corresponding pressure traces have been recorded. These pairs of frames show both the evolution of hot spots in the end-gas region as well as the evolution of the regular flame front at the border between unburned end gas and burned exhaust gas. Hot spot expansion speeds between 30 and 750 ms−1 have been detected, as well as flame front reverse speeds between 10 and 180 ms−1.