Recent topics of the use of reflectance difference spectroscopy (RDS) to control and characterize the Si-surface oxidation and the InAs wetting-layer formation are reviewed. It is shown that by measuring the RD-spectrum oscillation one can control the thickness of layer-by-layer Si-surface oxides in an atomic scale. Moreover, by analyzing the RD spectra, the atomic structure and component of the SiO₂/Si interface is characterized. On the other hand, the RD-spectrum change in the InAs growth on GaAs substrate before the quantum-dot formation enables us to clarify not only a variety of surface atomic structures of InAs wetting layers, such as (1×3), (2×3), and (2×4), but also the cation-alloying dynamic process of InAs wetting layers.