An experimental investigation was performed to determine critical semi-open channel height (h*) in which hydrogen-air detonation may propagate. Three types of gaseous mixture composition were used: 25%2 29.6%2and 40%2of hydrogen in air. Experimental setup was based on rectangular (0.11 x 0.11 x 2 m) test channel equipped with acceleration section (0.11 x 0.11 x 1 m) and data acquisition system with 5 pairs of pressure transducers and ion probes. Different channel heights h in range of 15 - 40 mm were used in the test channel. The critical height h* was defined for each investigated mixture. Additionally, sooted plates technique was used to determine detonation cell sizes and their relationship to h*. The results showed that detonation in stoichiometric H2-air mixture may propagate in semi-open channel only when the channel height is equal to or exceeds 3 . For less reactive mixtures this critical relation reaches 3.1 or 3.6 for mixtures with 25%2and 40%2of hydrogen in air respectively.