[en] We have established a novel reaction process using Mg vapour and B powder (a gas–solid reaction) to synthesize MgB₂ powder with a low oxygen content. For the first time, we tried to eliminate the oxygen in Mg powder with a specially designed furnace tube that can keep the sample under an inert atmosphere throughout the entire synthesis process. The duration of heat treatment has a conspicuous effect on the phase formation of MgB₂ which differs greatly from the conventional solid state reaction method. At short durations such as 1 h, unreacted B powder is ubiquitous in the sample. Full formation of MgB₂ is achieved at 3 h. At longer times such as 4 h, MgB₂ decomposes to form MgB₄. It is deduced that the main reason for the unstable MgB₂ phase formation in the gas–solid reaction is the Mg vapour present during the entire process. A feasible chemical reaction model of the novel gas–solid reaction has been established. This work provides an important foundation to obtain almost oxygen-free MgB₂, which can finally resolve the mystery of the effect of oxygen on superconductivity in this system. (paper)