The dissociative photoionization of CH₄ and CD₄ has been studied using synchrotron radiation within the energy range of 12-60 eV. The mass spectra of secondary CH_n⁺/CD_n⁺ 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 have 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.