In this paper, the reaction mechanisms of metalorganic chemical vapor deposition (MOCVD) have been investigated using infrared absorption spectroscopy. The growth of from Ga and under gas atmosphere in a hot wall reactor was studied. The compositions of gases which are sampled through a quartz capillary are observed by infrared spectroscopy. Infrared spectra of , , and systems were measured in the range from room temperature to 930°C. In the system, a new absorption peak at 2080 cm−1 which exists in neither the nor the system is observed. In the system, when is added into the reacting gas, the concentration of decreases drastically. The decomposition of is affected strongly by the addition of .