Coke formation on quartz surfaces during the pyrolysis and oxidative pyrolysis of CH3Cl, CH4, and their mixtures were studied using a gravimetric microbalance in the temperature range 1020-1260 K and at 1-atm pressure. The effects of process variables such as the temperature and gas composition (N2 dilution, CH4/CH3Cl ratio, and O2 concentration) on the rate of formation of coke has been determined. The presence of autocatalysis was indicated in both CH4 and CH3Cl pyrolysis. In addition, the effects of operating variables on the morphology of coke were also explored using scanning electron microscopy (SEM). The presence Of 02 in the system suppressed coke formation in all the mixtures studied. Both the thermogravimetric analysis and SEM studies indicated that coke formation in the pyrolysis of CH4 occurs via a single complex mechanism. In contrast, CH3Cl pyrolysis studies suggested the presence of at least two mechanisms of coke formation. X-ray elemental analysis also indicated that no chlorine was incorporated into the coke formed in the pyrolysis of CH3Cl.