Fast real‐time ellipsometry is used to studyinsitu, as a function of the substrate, the growth of the first tens of monolayers of hydrogenated amorphous silicon (a‐Si:H) deposited by a rf glow discharge of SiH4. The high sensitivity of this technique is illustrated and the early stage of the growth is found to strongly depend upon the nature of the substrate. A nucleation mechanism followed by incomplete coalescence is observed on metal and hydrogenated amorphous germanium (a‐Ge:H) substrates. On the contrary, fused silica (SiO2) and tin dioxide (SnO2) are superficially reduced:  this reduction creates at the interface a mixed layer ofa‐Si:H and silicon oxide on the silica substrate, and produces elemental tin at the surface of the SnO2substrate. In this last case, tin is found to diffuse in the furthera‐Si:H growing film. On crystalline silicon (c‐Si), thea‐Si:H growth shows incomplete coalescence followed by homogeneous growth, probably together with the reduction of the nativec‐Si oxide layer.