Ellipsometry signal oscillations were clearly seen during metalorganic vapor-phase epitaxy (MOVPE) of III-V materials under high sample rotation at 1400rpm. The ellipsometric signal was found to oscillate at a period corresponding to 1 monolayer of MOVPE growth. Oscillations correlated with the formation of islands due to nucleation on terraces during the growth of 1 monolayer. Layer thickness and composition of AlGaAs and InGaAs 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.