The dissociative photoionization of methane in the VUV AM Sands, RA Mackie, N Kouchi+, KF Dunn and CJ Latimer Department of Pure and Applied Physics, The Queen's University of Belfast,Belfast BT7 1NN, UK. +Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan The dissociative photoionization of CH4 and CD4 has been studied using synchrotron radiation within the energy range 12- 60eV. The mass spectra of secondary ions have been found to display significant isotope effects and to depend on the target gas temperature. The kinetic energy spectra of the fragment protons and deuterons has also been determined and it is shown that , in addition to direct photoionization, intermediate superexcited states play an important role in the photodissociative ionization process.