Oscillations of the ellipsometry signal were clearly seen during metalorganic vapor-phase epitaxy (MOVPE) of III-V materials under high sample rotation at 1400 rpm. The ellipsometric signal oscillated at a period corresponding to 1 monolayer of MOVPE growth. Agreement was excellent between growth rate data from the oscillation period and that from thickness measurements after growth(deviation: < 1%). Oscillations correlated with the formation of islands due to nucleation on terraces during the growth of 1 monolayer. The effective medium dielectric response of smooth and rough surfaces appears responsible for different ellipsometry signals. Layer thickness and the InGaAs composition were precisely determined in situ. Critical layer thickness and In composition for InGaAs quantum dot formation were also evaluated directly from in situ ellipsometry data.