The unintentional and intentional incorporation of carbon has been investigated in order to understand both the growth kinetics as well as develop new and useful carbon doping sources. We present in this study a systematic investigation of the use of halomethanes during the MOVPE based growth of GaAs. We have used 13C tagged trimethyl gallium, AsH3, together with non-isotopically tagged halomethane sources, in order to determine the source and mechanism of the carbon incorporation. The carbon concentration was determined by secondary ion mass spectroscopy (SIMS). The carbon incorporation in the unintentionally doped material is compared to that obtained with the series of halomethanes, CHxI4-x (x=0,2-4), the chlorine analogs, CHC13 and CCI4, and the bromine containing compounds, CHBr3 and CBr4 which are used to intentionally introduce carbon in the epitaxial GaAs. The incorporation of carbon is studied here as a function of the halogen/H ratio in the source material and on the halogen ligand on the carbon nucleus. The standard growth parameters of susceptor temperature and gas phase composition were varied in order to discern the useful growth parameter regimes for these compounds.