A remote plasma chemical vapour deposition (RPCVD) system for the growth of gallium nitride (GaN) thin films is investigated using optical emission spectroscopy (OES). The intensities of the various excited species in pure nitrogen as well as nitrogen/hydrogen plasmas are correlated with GaN film growth characteristics. We show a correlation between the plasma source spectrum, the downstream spectrum where trimethylgallium is introduced and the GaN film quality. In particular, we investigate the addition of hydrogen, which greatly affects the gas phase species and the GaN film characteristics. OES is demonstrated to be a valuable monitoring tool in a RPCVD system for optimization of GaN growth.