Ellipsometric measurements on single ultrathin InP layers of 0.5 and 1 monolayer (ML) thickness inserted into GaP by metal-organic vapor phase epitaxy (MOVPE) growth have been carried out at room temperature in the blue spectral range (hv = 2.5 to 2.9 eV) around the Gap direct ap. Due to the insertion of the InP monolayers, the optical dielectric function of the GaP host material is strongly modified in dependence on the InP layer thickness.