Spectroscopic ellipsometry and secondary ion mass spectrometry have been performed on structures consisting of a microcrystalline silicon film deposited on different a-Si:H substrates. The substitution of hydrogen by deuterium in the microcrystalline growth allowed us to quantify the long-range effects of hydrogen and to distinguish between the different layers. The thicknesses deduced from ellipsometry measurements were in excellent agreement with the secondary ion mass spectrometry profiles. Moreover, we clearly show that the a-Si:H film on which the microcrystalline silicon is deposited can be modified because of the diffusion of hydrogen necessary to the formation of the microcrystalline silicon. However, this modification strongly depends on the deposition conditions and abrupt interfaces can be achieved in some cases.