We have developed techniques to determine the near‐infrared to near‐ultraviolet dielectric function and optical gap of ultrathin amorphous silicon [a‐Si:(H)] using real‐time spectroscopic ellipsometry during preparation and processing. The techniques have been applied to ∼50 A˚a‐Si:H films prepared by plasma‐enhanced chemical vapor deposition, and to ∼250 A˚ purea‐Si chemically modified by atomic H exposure. For the latter, the time evolution of the bonded H content can be estimated along with the evolution of the gap.