The reactions of BH3·thf (thf = tetrahydrofuran) with selected (dialkylamino)dimethylarsines, Me2AsNR2 (R = Me, Et, Prn, or Pri) have been carried out as a function of temperature to determine the initial co-ordination site of boron and to elucidate the nature of any As-N, As-B, and N-B bond dissociation/formation processes in solution. The reactions were monitored by multinuclear (1H, 11B, and 13C) n.m.r. spectroscopy over a temperature range of -90 to 25°C. With Me2AsNMe2, a N-B bonded adduct is formed at low temperature, which decomposes at 25°C to yield (Me2NBH2)2, Me2NH·BH3, μ-Me2NB2H5, and Me2AsAsMe2. Equimolar amounts of As-B and N-B adducts are formed at low temperature when Me2AsNEt2 is reacted with BH3·thf. With increasing temperature, the As-B adduct converts to the N-B adduct, which subsequently decomposes at room temperature to μ-Et2NB2H5, Et2NH·BH3, (Et2NBH2)2, and Me2AsAsMe2. The reaction of Me2AsNPrn2 with BH3·thf yields more of the As-B adduct than of the N-B adduct at -90°C, but the former rearranges to the latter on warming. Only the As-B adduct is formed in the reaction of Me2AsNPri2 with BH3·thf. These results suggest that the steric bulkiness of the R2N moiety plays an important role in determining the co-ordination site of the boron in this series of compounds.