Many technologically important processes from ore smelting to hydrogen storage and the synthesis of functional materials rely on solid-gas reactions. Following these reactions in situ (under gas pressure and increased temperature) allows to unravel their basic steps and helps to gain better control over the processes. We have constructed a gas-pressure cell based on a single-crystal sapphire tube as sample holder for in situ studies. The key idea is to completely avoid background contribution of the container (sapphire) in the experiment by proper orientation of the cell. This will eliminate Bragg peaks from the container material and increase data quality considerably. If tested successfully, the single-crystal sapphire cell may be used at ISIS for in situ investigations on a variety of solid-gas reactions.