Gallium nitride and its alloys promise to be key materials for future semiconductor devices aimed at high frequency, high power electronic applications. However, manufacturing for such high performance products is challenged by reproducibility and material quality constraints that are notably higher than those required for optoelectronic applications. To this end, in situ mass spectrometry was implemented in AlGaN/GaN/AlN/SiC metalorganic chemical vapor deposition processes as a real-time process and wafer state metrology tool. Dynamic chemical sensing through the process cycle, carried out downstream from the wafer, revealed generation of methane and ethane reaction byproducts, as well as other residual gas species. Using the methane/ethane ratio, the GaN epilayer crystal quality was shown to be predictable in real time to a precision of 2%–5%. This was verified by postprocess x-ray diffraction using the full-width at half-maximum height of GaN on-axis (002) and off-axis (102) rocking curve peaks as a measure of crystal quality. The methane/ethane ratio may have a fundamental significance in terms of the intrinsic chemistry in that these two byproducts are speculated to reflect two different reaction pathways leading to GaN growth, namely the gas phase adduct formation route and the gas phase thermal decomposition of the precursor, respectively. The fact that lower methane/ethane ratios consistently yield better crystal quality for the GaN films suggests that the gas phase thermal decomposition pathway produces higher quality GaN growth. These results demonstrate that in situ mass spectrometry can be used to predict material quality during crystal growth. In turn, this offers an attractive pathway to advanced process control for GaN-based semiconductor manufacturing.