A novel experimental scheme for probing mode-specific proton transfer in proton-bound homodimers based on the vibrational predissociation spectroscopy combined with the techniques of mass-selected detection and isotope substitution is described. Its application to CH5+.CH4 suggests that rapid, non-mode specific proton transfer occurs when three C-H stretch modes are excited. To explain the experimental result, the transition state and the minimum energy path (MEP) for proton transfer in CH5+.CH4 are further characterized by ab initio method. (C) 2003 Elsevier Science B.V. All rights reserved.