To gain insight into the mechanism of turbulent flame propagation, a technique using imaged Rayleigh scattering has been developed to directly measure turbulent flame thickness in an operating engine. Spatial resolution of 0.1mm and time resolution of 0.01 microsecond were achieved by using a multielement detector and a pulsed laser. The use of two laser pulses to simultaneously measure flame thickness and propagation speed was also demonstrated.Mean flame thickness was found to increase gradually from slightly greater than laminar values at very low rpm to two to three times the laminar value at 1800 rpm. The standard deviations of the thickness distributions increased with increasing engine speed. There was also an increasing probability of small “islands” of unburned mixture being found immediately behind the flame front as rpm increased. Variations in fuel and equivalence ratio were found to have a small effect on thickness.