Multipacting induced discharge in a rectangular waveguide is studied experimentally at a RF frequency of 500 MHz. The waveguide has a cross-section of 457 mm by 102 mm. The maximum forward RF power is 600 and 300 kW in traveling and standing wave mode, respectively. A continuous multipacting band structure is observed. Electron pick-up probes of antenna-type are used to measure the multipacting current and its longitudinal as well as horizontal distributions. The electron energy spectrum is measured with the retarding field method. The end-point energy of the spectra taken in traveling wave mode is in the range of 100-1000eV and agrees well with the impact energy calculated by the classical multipacting theory. However, a large fraction of electrons has energies lower than 100eV. Electron stimulated gas desorption is found to play a critical role in the dynamics of multipacting induced breakdown. It is concluded that the ionization discharge of desorbed gases is the immediate cause for RF breakdown. (C) 2004 Elsevier B.V. All rights reserved.