The deposition of a-Si:H films on crystalline silicon substrates was monitored in situ by transient photoconductivity measurements in the microwave frequency range. At the start of the deposition a drastic increase of the interface recombination velocity was observed, followed by a rapid decrease. The implications of these results for the structure of the interface are discussed. Changes of the interface after deposition were detected without a change of the temperature, even at 250'C: The long relaxation time of the structure of the interface will be discussed. Ex situ results on the samples produced will be compared to the in situ results in view of the passivation properties of a-Si:H films on c-Si substrates.