The dehydrocoupling/dehydrogenation behavior of primary arylamine–borane adducts ArNH2⋅BH3 (3 a–c; Ar=a: Ph, b: p-MeOC6H4, c: p-CF3C6H4) has been studied in detail both in solution at ambient temperature as well as in the solid state at ambient or elevated temperatures. The presence of a metal catalyst was found to be unnecessary for the release of H2. From reactions of 3 a,b in concentrated solutions in THF at 22 °C over 24 h cyclotriborazanes (ArNH-BH2)3 (7 a,b) were isolated as THF adducts, 7 a,b⋅THF, or solvent-free 7 a, which could not be obtained via heating of 3 a–c in the melt. The μ-(anilino)diborane [H2B(μ-PhNH)(μ-H)BH2] (4 a) was observed in the reaction of 3 a with BH3⋅THF and was characterized in situ. The reaction of 3 a with PhNH2 (2 a) was found to provide a new, convenient method for the preparation of dianilinoborane (PhNH)2BH (5 a), which has potential generality. This observation, together with further studies of reactions of 4 a, 5 a, and 7 a,b, provided insight into the mechanism of the catalyst-free ambient temperature dehydrocoupling of 3 a–c in solution. For example, the reaction of 4 a with 5 a yields 6 a and 7 a. It was found that borazines (ArN-BH)3 (6 a–c) are not simply formed via dehydrogenation of cyclotriborazanes 7 a–c in solution. The transformation of 7 a to 6 a is slowly induced by 5 a and proceeds via regeneration of 3 a. The adducts 3 a–c also underwent rapid dehydrocoupling in the solid state at elevated temperatures and even very slowly at ambient temperature. From aniline–borane derivative 3 c, the linear iminoborane oligomer (p-CF3C6H4)N[BH-NH(p-CF3C6H4)]2 (11) was obtained. The single-crystal X-ray structures of 3 a–c, 5 a, 7 a, 7 b⋅THF, and 11 are discussed.