The intramolecular vibrational relaxation (IVR) of an excited Si-H stretch (second overtone) and C-H stretch (first overtone) in methylsilane has been examined by eigenstate resolved infrared spectroscopy. The experiment probes a molecular beam produced in a supersonic expansion, excited by a laser in a power buildup cavity, and detected by a liquid helium cooled silicon bolometer. The Si-H stretch [local mode (3,0,0), both A and E combinations] is compared with the nearly isoenergetic C-H stretch [predominantly the 2 nu(7)(0) band]. With the calculated density of states almost unchanged, the two modes exhibit very different IVR behavior, which is quantified in terms of the lifetime of the bright states and the coupling between the bright states and the dark states. (C) 1997 American Institute of Physics. [S0021-9606(97)00541-2].