In the transition to a hydrogen economy it is likely that hydrogen will be used or stored in close proximity to other flammable fuels and gases. Accidents can occur that result in the release of two or more fuels, such as hydrogen and natural gas, that can mix and form a hazard. A series of five medium-scale semi-open-space deflagration experiments have been conducted with hydrogen, natural gas, and air mixtures. The natural gas consisted of 90%2methane, 6%2ethane, 3%2propane, and 1%2butane by volume. Mixtures of hydrogen and natural gas were created with the hydrogen mole fraction in the fuel varying from 1.000 to 0.897 and the natural gas mole fraction varying from 0.000 to 0.103. The hydrogen and natural gas mixture was then released inside a 5.27-m3 thin plastic tent. The stoichiometric fuel-air mixtures were ignited with a 40-J spark located at the bottom center of the tent. Overpressure and impulse data were collected using pressure transducers located within the mixture volume and in the free field. Flame front time-of-arrival was measured using fast response thermocouples and infrared video. Flame speeds, relative to a fixed observer, were measured between 36.2 m/s and 19.7 m/s. Average peak overpressures were measured between 2.0 kPa and 0.3 kPa. The addition of natural gas inhibited the combustion when the hydrogen mole fraction was less than or equal to 0.949. For these mixtures there was a significant decrease in overpressures. When the hydrogen mole fraction in the fuel was between 0.999 and 0.990, the overpressures were slightly higher than for the case of hydrogen alone. This could be due to experimental scatter, or there may be a slight enhancement of the combustion when a very small amount of natural gas was present. From a safety standpoint, variation in overpressure was small and should have little effect on safety considerations.