Ultrahigh vacuum evaporation of magnesium onto a hot silicon substrate (⩾200 °C), with the intention of forming aMg2Sithin film by reaction, does not result in any accumulation of magnesium or its silicide. On the other hand,codeposition of magnesium with silicon at 200 °C, using a magnesium-rich flux ratio, gives a stoichiometricMg2Sifilm which can be grown several hundreds of nm thick. The number of magnesium atoms which condense is equal to twice the number ofsiliconatoms which were deposited; all the silicon condenses while the excess magnesium in the flux desorbs. TheMg2Silayers thus obtained are polycrystalline with a (111) texture. From the surface roughness analysis, a self-affine growth mode with a roughness exponent equal to 1 is deduced. ©1997 American Institute of Physics.