In this paper the effect of concentration gradients on peak pressures measured during explosive combustion of hydrogen-air mixtures is investigated. Reflection of shock waves at the end wall of a closed explosion channel with subsequent shock-flame interaction causes deflagration-to-detonation transition. Concentration gradients can cause this phenomenon to appear at distinctly lower average hydrogen concentrations than in homogeneous mixtures due to faster flame acceleration. This implies that pressure loads during explosive combustion of hydrogen-air mixtures with concentration gradients cannot be predicted by concepts considering a homogeneous mixture as worst-case scenario.