An ultrahigh vacuum compatible source of methyl radicals has been developed and used to investigate the initial stages of SiC growth on Si(100) 2x1 surfaces. This source produces a pure, high flux of methyl radicals ideally suited for film growth and surface kinetics studies. For comparable growth conditions, our studies reveal that radicals exhibit a propensity to maintain SIC stoichiometry, while molecular species (ethylene) tend to rapidly produce carbon-rich films. Films grown using radicals and molecular species also differ in microstructure. Methyl radicals produce films which have extremely smooth surfaces, while those films grown using molecular species exhibit hillocks or small grains.