The detonations propagating through the annular channel of an optically accessible rotating detonation engine (RDE) operating on hydrogen-air are visualized using OH* chemiluminescence imaging. The images are useful for observing the instantaneous size and shape of the detonation structure, oblique shock wave, and possible presence of deflagration between the fuel-fill zone and expansion region containing detonation products. The detonation increases in height as the air flow rate is increased for low flow rates, experiences subtle changes for intermediate flow rates, and transitions from one to two waves for higher flow rates. The two detonation waves typically propagate in the same azimuthal direction. Counter-rotating waves resulting in detonation-detonation interactions are observed for some configurations with a reduced number of fuel injection jets. Time-dependent static pressure measurements show that acoustic interactions between the detonation channel and air plenum are important for low air flow rates and large air injection areas. The OH* chemiluminescence images, pressure, and wave speed measurements provide benchmark data that are useful for evaluating RDE models and simulations, improving fundamental understanding of the detonation structure in RDEs, and identifying critical design parameters that influence RDE operation and performance. Published by Elsevier Inc. on behalf of The Combustion Institute.