Details are given for the preparation of the monomeric aryl- and diaryl-methyleneaminoboranes Ph2C:NBPh2, p-tolyl2C:NBPh2, (p-CIC6H4)2C:NBPh2, p-BrC6H4(Ph)C:NBPh2, Ph2C:NB(mesityl)2, PhCH:NB(mesityl)2, and 2,2′- biphenylylC:NBPh2 by one or more of the following routes: (1) R12C:NSiMe3+ R22BX; (2) R12C:NLi + R22BX; (3) 2R12C:NH + R22BX (4) R12C:NBX2+ 2R2Li (5) R12C:NH2+Cl-+ NaBPh4(X = halogen). The intermediate adduct (p-tolyl)2C:NH,BPh3 was isolated in a reaction of type (5). The associated alkylideneaminoboranes (Ph2C:NBPhCl)n, (PhCH:NBPhCl)2, (PhCH:NBPh2)2, [Ph2C:NB(2,2′-biphenylyl)]n, and (Ph2C:NBO2C6H4)n were similarly prepared. The monomeric alkylideneamino-boranes are apparently prevented from dimerising by the bulk of the substituents, particularly on boron, which also inhibit approach of potential donor molecules. Except for the air-stable Ph2C:NB (mesityl)2, they are hydrolysed by moist air. Their i.r. spectra have characteristic absorptions, assigned ν(CNB), in the range 1765–1820 cm.–1. 1H n.m.r. and mass spectroscopic details of several of the compounds are given and discussed.