Reduction of the trifluoroacetylindoles 1-5 with diborane affords the hydroxytrifluoroethylindoles 7-10 (20-70%), the trifluoroethylindoles 6 and 11 (8.4-18.5%) and the unique and dimeric indolinotrifluoroethylboranes 16, 17a and 17b (0.6-12%), This appears to be the first report on the formation of alkylboranes in the reduction of a carbonyl chromophore. The existence of indolinoalkylboranes in the form of the dimers 16, 17a and 17b is confirmed by 11B NMR and mass spectra, which show molecular ions for both the monomers and the dimers. Compound 5 does not form dimers corresponding to 16 and 17,probably due to the electron-withdrawing effect of its phenyl group. The results have been discussed in relation to the mechanisms of diborane reduction and the origin of different products.