The pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectra of CH4 and CD4 have been recorded in the region 100880 - 104100 cm(-1). From the analysis of the photoelectron spectra the first adiabatic ionization potential of CH4 and CD4 has been determined to be (101773 +/- 35) cm(-1) and (102210 +/- 25) cm(-1), respectively. A one-dimensional model for the pseudorotation between three equivalent C-2 upsilon equilibrium structures shows evidence for a fluxional behavior of CH4+.