THE discovery(1-3) of the stoichiometric compounds He(N-2)II, Ne(He)(2) and Ar(H-2)(2) in high-pressure mixtures has uncovered a new aspect of high-pressure chemistry. In contrast to these studies on inert species, we show here that high pressures can lead to unexpected behaviour in reactive compounds. We find that mixtures of O-2 and H-2 are stable at pressures of around 7.6 GPa, despite their explosive nature under ambient conditions. We use microscopy and Raman spectroscopy to identify the phase boundary of the stable region. The structure of the binary phase diagram at 296 K suggests that a stoichiometric compound (O-2)(3)(H-2)(4) might exist. The stability of dense O-2/H-2 mixtures might be usefully exploited in the development of rocket fuels and for energy storage, and might also be relevant to the composition of the interiors of the jovian planets.