The structure and mode of propagation of spinning detonation waves in stoichiometric oxyhydrogen, at initial pressures of 20–30 mm, have been investigated. The waves were generated in a square-section tube and observations have been made by the smoked-film technique, spark schlieren photography and pressure gauges. At the front of the self-sustaining detonation waves obtained at these pressures, two Mach interactions exist, the trajectories of which are derived from the imprints made on the smoked foil. As the triple point traverses the tube section, its direction of motion is found to vary between 50° and 70° with the tube diameter. An analysis of a Mach triple point for these conditions predicts the absence of chemical reaction behind the Mach stem in the immediate neighbourhood of the triple point. Experimentally determined pressures and triple shock angles confirm, to within experimental error, the postulated theoretical configuration.