We have employed multi-reference CI calculations to investigate in some detail the CH3C to CH2CH isomerization. By following the motion of an H atom along the C-C axis, optimized potential energy curves have been constructed for three low lying states of CH3C (X(2)A", A(2)A', (a) over tilde (4) A") and connected to optimized potential energy curves of correlated CH2CH states. The isomerization barrier for each CH3C state is estimated from these curves, and the feasibility of the formation of the C2H3O+ ion is discussed. The ab initio energies and geometry changes along the isomerization paths are given, to help visualize the isomerization process. (C) 2002 Elsevier Science B.V. All rights reserved.