The effects on the title reaction of collision energy (E-col), five H2CO+ vibrational modes, and deformation vibrations of methane have been studied, including the measurement of product integral and differential cross sections over a center-of-mass E-col range from 0.09-3.3 eV. Electronic structure and RRKM calculations are reported, providing an additional mechanistic insight. The total reaction efficiency is well below unity, despite there being two exoergic reaction pathways with no activation barriers. The energetically more favorable channel corresponds to H elimination (HE) from an intermediate complex, however, this channel accounts for only similar to15% of the total reaction cross section at low E-col and is negligible at high energies. The dominant channel, hydrogen abstraction (HA) by H2CO+ from methane, is dominated by a complex-mediated mechanism at low E-col, switching over to a direct hydrogen-stripping mechanism at high E-col. Both HA and HE are inhibited in a strongly mode-specific fashion by H2CO+ vibrational excitations, and greatly enhanced by excitation of methane deformation vibrations. The strong mode specificity indicates that the reaction-limiting step occurs early in the collisions. (C) 2003 American Institute of Physics.