Combustion during normal and knocking cycles has been observed in a single cylinder, spark ignition engine. Photographs of combustion in a disc chamber showed that autoignition of the end gas was the cause of knock. Normal flame propagation was found to decelerate more quickly becoming stationary about two degrees before autoignition became visible.By contrast flame propagation was similar for normal and knocking cycles in a compact chamber. However pressure rises in excess of 8 bar/degree at 1500 rpm occurred during knocking cycles. It was concluded that knock resulted from rapid entrainment and burning of reactants by the normal spark ignited flame.