Abstract: The reaction zone structure of gaseous detonationwave in stoichiometric hydrogen-oxygen mixture diluted with 76.92\% argonwas directly observed by planar laser induced fluorescence technique. Suchhigh argon dilution was employed to degenerate natural fluorescence mainlycaused by detonation chemical reaction. The synchronization of the PLIFsystem, the detonation wave and ICCD were carefully adjusted. The images ofOH fluorescence distribution in the wake of the detonation front were obtained.These images show that the reaction zone is spatially and temporallychangeable and the reaction front is not planar and is very unstable. Thedistinct keystone-like structure, with the incident shock,the shear layers and the reaction front as its boundaries, can be clearly observed. Whetherthe OH concentration is behind the Mach stem or the incident shock, itabruptly increases to the maximum value at the end of the induction zone andthen decreases with increase of the distance from the reaction front. Due tothe effects of the detonation mode and laser sheet direction, the transversewave spacing measured from the PLIF images exhibits a relatively largescatter, and assumes a value smaller than the cell width.