The reaction of H atoms with methylacetylene was investigated in an isothermal flow system at 195-503 DegK/1-18 Torr. Stable products (H2, C3H6, allene, C2H2, CH4, C2H6, C2H4, C3H8 and small amts. of isobutene, 2,3-dimethyl-1,3-butadiene, 1-butene) were detd. by gas chromatog. and mass spectroscopy. The product distribution can be best explained assuming primary addn. of H atoms to either terminal (1) or non-terminal (2) position in methylacetylene: H + MeC.tplbond.CH -> .bul.CMe:CH2* (1); H + MeC.tplbond.CH -> MeCH:H*C.bul. -> Me.bul. + C2H2 (2). H-atom abstraction from the Me group is not observed. By measuring the H-atom concn. using ESR spectroscopy, the rate consts. k1 and k2 could be evaluated sep. to give: k1 = (6.5 +- 1.2) 1012 exp[-[(2000 +- 200) cal mole-1]/RT] cm3/mole sec; k2 = (5.8 +- 1.2) 1012 exp[-[(3100 +- 250) cal mole-1]/RT] cm3/mole sec.