Experiments are described in which hydrogen was released at the center of the floor of a real-scale enclosure having dimensions of a typical two-car residential garage. Real-time hydrogen concentrations were monitored at a number of locations. The hydrogen/air mixtures were ignited at pre-determined volume fractions ranging from 8 %2to 29 %2 The combustion behavior and structural effects were monitored using combinations of high-speed pressure transducers and ionization gauges, standard thermocouples, hydrogen sensors, and digital, infrared, and high-speed video cameras. Experiments were performed both for empty garages and garages with conventional automobiles parked above the hydrogen release location.