A real-time spectroscopic ellipsometer (RTSE) was designed and implemented on an oxide molecular beam epitaxy (MBE) system. The RTSE was designed as a complementary tool to the other existing in situ deposition monitors on the MBE. To quantify how the RTSE complemented the other tools las well as to determine its limitations), the RTSE was used to characterize the deposition of (111)-oriented Y2O3 on (111) Si and (110)-oriented Y2O3 on (100) Si. Results from computer modeling of the RTSE data subsequent to deposition showed excellent agreement with atomic absorption flux measurements, quartz crystal monitor flux measurements, refection high energy electron diffraction measurements, and Rutherford backscattering spectroscopy. From the RTSE measurements, growth rates and microstructures were determined and verified by ex situ techniques. In addition, the sticking coefficient of yttrium to Y2O3 was found to be 1.00 +/-0.07. Also, the temperature dependent optical properties of the Y2O3 films were measured at 25 and at 730 degreesC. Nearly bulk values were found, indicating the high quality films deposited via this method.