Mg₂Si composite was added to Al-Si filler metal for aluminium alloy vacuum brazing via in situ synthesis. Spreading area method was used to measure the wettability of the brazing alloy on the parent metal in vacuum. The filler metal wetting area increased with increasing Mg₂Si content. The compound within the interface was examined with an electron probe and the following results were obtained: Mg₂Si content of the brazing interface was lower than that of filler metal, Mg₂Si was distributed into both sides of the interface and arranged in block, grain and short rod-like shapes. Si crystal had a primarily needle-like structural arrangement. The elemental distribution on both sides of the interface was analysed using thread scanning method. A solid solution of Mg was formed in the interior parts of the grain in the parent metal. The solid solution content decreased as the distance from the interface increased.