A photographic investigation of combustion in gases was undertaken to observe experimentally the mechanism of initiation and structure of detonation waves. Streak schlieren photographs of hydrogen‐oxygen and carbon monoxide‐oxygen explosions were made in a rectangular shock tube using precision optics and a rotating mirror camera.The photographs show that compression waves generated during the early stages of propagation of the explosion soon steepen into a shock wave which travels ahead of the combustion wave (zone of chemical reaction). This combination of shock and combustion wave is characteristic of a nonsteady detonation state in the moments before stable detonation is initiated. The distance between the two waves narrows as the explosion progresses down the tube due to the continued formation of new combustion centers ahead of the combustion wave by means of auto‐ignitions. Eventually the combustion wave overtakes the shock wave and supplies it directly with energy from the chemical reaction; at this moment stable detonation is initiated.The internal structure of steady state detonations is found to be different from nonsteady state detonations; for example, whereas the distance between the shock and the zone of chemical reaction is of the order of several centimeters in the nonsteady state detonation, this distance in the steady state detonation is too small to be resolved by the optical system (i.e., less than 1 mm). Other structural features such as spinning detonation and a high‐frequency oscillation of propagation velocity are also illustrated.