Ellipsometric measurements have been made at roam temperature on MOVPE-grown InAs monolayers (ML) (0.5-1 ML) and AlAs MLs (1 to 12 ML) in GaAs around the GaAs fundamental Sap E-0(GaAs) and on MOVPE-grown InP MLs (0.5-1 ML) in GaP around the GaP direct gap. Due to the introduction of monolayer films, the optical properties of the host material are strongly modified as a function of the monolayer material and thickness and as a function of the substrate misorientation. The perfection of the steplike monolayer/host material interfaces has been examined hy high-resolution transmission electron microscopy (HRTEM). To our knowledge the modifications of the optical properties of the system InP in GaP have been observed only by ellipsometric measurements.