High-sensitivity proton efficiency functions have been measured using a monoenergetic election beam impacting CH4 under single-collision conditions_ The initial H+ onset energy and structure in the threshold region strongly support the interpretation that proton productlon at threshold is dominated by dissociative autoionisation involving specific Rydberg states of neutral CH4. Though dissociative ionisation of methane has attracted attention over a number of years (see ref. [l] for the most recent tabulation of pertinent data), the tendency has been to concentrate efforts on studying the higher mass fragment ions. As a result there remains a paucity of experimental data concerning proton formation by electron and photon impact. The earliest mass spectrometric measurements were carried out over four decades ago [2] and yielded a value of 22.7 + 0.5 eV for the lowest appearance energy of Hf from methane. It is only in the last decade that further attempts have been made to remeasure this onset in order to understand the energetics and kinetics of fragmentation processes in CH4. The electron impact experiments of Appell and Kubach [3] and Locht et al. [4] yielded lowest appearance energies of 24.0 f 0.5 eV and 21.3 + 0.3 eV, respectively. Corresponding photon impact onsets have been determined to be e-21 eV using photoionisation mass spectrometry [S] and 22.2 eV using synchrotron radiation [6] _ Similar differences exist in the various values ascribed to higher onsets, as can be seen from table 1 which summarises the current, rather unsatisfactory, state of affairs. In an effort to resolve the discrepancies we report results of what we believe to be the first measurements of the threshold appearance energy function of H+ resulting from monoenergetic electron