The effects of hydrogen on carbon incorporation in GaN layers grown by remote plasma-enhanced metal-organic chemical vapor deposition were investigated. Gas-phase decomposition of TEGa was enhanced at high RF power and reactor pressure conditions. With increase in hydrogen fractions in the downstream plasma, optical emission intensity of CN abruptly decreased. It is suggested that active hydrogen species may scavenge the carbon-containing species in the gas phase, resulting in reduced carbon incorporation in GaN layers measured by 10K photoluminescence and secondary-ion mass spectroscopy.