Bis(sulfurdiimido)tin compounds of the type R2Sn[(NSN)tBu]2 [R = Me (2a), Et (2b)] re act with triethylborane Et3B by rearrangement and N-ethylation to give the novel aminoborane derivatives R2Sn[N(tBu)SN(Et)BEt2]2 (5a, b), in the same way as nBu3Sn(NSN)tBu (1) reacts with Et3B to give nBu3Sn[N(tBu)SN(Et)BEt2] (4). In contrast, 2c (R = tBu) and tris(sulfurdiimido)tin compounds RSn[(NSN)tBu]3 [R = Me (3a), nBu (3d)] react with Et3B mainly by Sn- and S-ethylation, accompanied by extensive decomposition. 1H, 11B , 13C, 15N and 119Sn NMR spectroscopy served for the structural assignment in solution.