The emergency venting of a gaseous deflagration is widely used to prevent severe consequences of a confined explosion. Processes of vented deflagrations have been studied extensively and a substantial amount of experimental data has been analyzed. It has been found that venting, in some cases, can result in strong explosions. One of the reasons is flame instabilities, which give rise to flame area increase and the generation of high peak overpressures. The purpose of the present paper is to report observations of detonation onset in the course of hydrogen-air deflagration, due to sudden venting. Although the aim of the tests was not to investigate the effect of venting on gaseous explosions, this effect appeared to be defining the explosion mode. The tests were in the modified KOPER facility, as an extension of unconfined turbulent jet initiation experiments. Test variables were hydrogen concentration, jet orifice diameter, and size of the enclosure. Measurement systems included pressure transducers, photodiodes and two high-speed cameras (2,500 frames/s). Contact gauges gave the times of jet membrane rupture and of panels destruction.