The steric opacity functions for the title Penning ionization reactions were determined. It was found that the most reactive site shifts from the collinear X-end toward the sideways direction, as the X change from Cl to I. The main feature in the experimental steric opacity function can be well explained by the electron exchange mechanism, which is controlled by the shift of center-of-mass and the electron density distributions of the HOMO of CH3X molecule. (C) 2002 Published by Elsevier Science B.V.