We have verified theoretically the assumption advanced by experimental groups that the CH3C radical in its (a) over tilde (4)A(2) excited state is one of the possible precursors of the CH3CO+ ((X) over tilde (1)A(1)) chemi-ion generated in the oxidation of 2-butyne. At the same time we have showed that the ground state of the CH3C radical cannot be such a precursor because of the existence of a potential barrier along the reaction coordinate in combination with its fast isomerization to CH2CH. (C) 2003 Elsevier Science B.V. All rights reserved.