Ion-molecule reactions of diethylsilylium ions with hexamethyldisilazane and isobutylamine were studied radiochemically. These reactions, like the previously studied reactions of diethylsilylium ions with tert- butylaminotrimethylsilane, occur exclusively along the condensation pathway; in both cases, the diethylsilylium ion undergoes a rearrangement. The extremely low degree of rearrangement of the diethylsilylium ion and high yield of the labeled substrate in the reaction with isobutylamine are due to the presence of two labile N-H hydrogen atoms, resulting in active isotope exchange between the Si-T tritium atoms and N-H hydrogen atoms and in shorter lifetime of the condensation complex, caused by more intense bimolecular deprotonation of the complex.