Lean isobutane and n-butane hat lean flames burning at low pressure have been investigated by means of molecular beam mass spectrometry. Profiles of stable, atomic and radical species have been measured as well as temperature. Acetone and butene occur as intermediates in significant amount. - In isobutane flames the production of both acetone (C3H6O) and butene (C4H8) is ascribed to direct reactions of butyl radicals t-C4H9 + O --> C3H6O + CH3 C4H9 + H, OH --> C4H8 + H-2, H2O t-C4H9 --> C4H8 + H - In n-butane flames, acetone is not produced and butene is accounted for along radical reactions C4H9 + H, OH --> C4H8 + H-2, H2O The occurrence of radical-radical reactions in higher hydrocarbons flames cannot be neglected and supplements the usual thermal decomposition reaction (beta-scission) in the detailed combustion mechanisms.