Abstract: Systems and methods for in-situ monitoring of the formation of parasitic particles during the deposition of a III-V nitride film with, e.g., metal-organic chemical vapor deposition (MOCVD) are described. In accordance with certain embodiments, at least one light source capable of generating a light beam at a desired wavelength is positioned relative to a reaction chamber so as to pass a light beam into the reaction chamber. Multiple optical detectors capable of detecting light from the beam are positioned relative to the reaction chamber to monitor desired reaction and growth conditions. More particularly, a first optical detector is positioned so as to detect light reflected from a deposition surface within the reaction chamber so as to monitor growth rate and/or composition of a film during deposition.