The reactions of BH,.thf (thf = tetrahydrofuran) with selected (dialkylamino)dimethylarsines, Me,AsNR, (R = Me, Et, Pr", or Prl) have been carried out as a function of temperature to determine the initial co-ordination site of boron and t o elucidate the nature of any As-N, As-B, and N-B bond dissociation/formation processes in solution. The reactions were monitored by rnultinuclear (IH, "B, and 13C) n.m.r. spectroscopy over a temperature range of -90 to 25 "C. With Me,AsNMe,, a N-B bonded adduct is formed at low temperature, which decomposes at 25 "C to yield (Me,N B H, ), , Me,N H-BH,, p-Me,N B, H, , and Me,AsAsMe,. Equimolar amounts of As-B and N-B adducts are formed at low temperature when Me,AsNEt, is reacted with BH,-thf. With increasing temperature, the As-B adduct converts t o the N-B adduct, which subsequently decomposes at room temperature to p-Et,NB,H,, Et,NH-BH,, (Et,NBH,),, and Me,AsAsMe,. The reaction of Me,AsNPrn, with BH,-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 Me,AsNPr+, with BH,ethf. These results suggest that the steric bulkiness of the R, N moiety plays an important role in determining the co-ordination site of the boron in this series of compounds.