Stoichiometry control of lattice matched compound semiconductors during growth is still a challenge for in situ optical techniques. This is because the energy shifts of the semiconductor optical gap induced by strain and stoichiometry compensate one another. Thus conventional critical point analysis for composition measurement is practically impossible. The remaining response of the measured ellipsometric parameters is very small but can be resolved when the signal-to-noise ratio of the ellipsometer is sufficiently high. We exploit this for the closed-loop control of InxGa1-xAs growth on InP where the trimethylindium (TMIn) flux was directly controlled by the measured optical response in order to reach the desired lattice matched composition (x = 0.53). Es-situ X-ray diffraction analysis verified ellipsometrically controlled lattice matched growth.