The new mono-B-alkylcyclotriborazanes 2-(PhC(2)H(4))B3N3H11 and 2-(C2H5)B3N3H11 have been synthesized by reaction of the mono-B-alkylborazines 2-(PhC(2)H(4))B3N3H5 and 2-(C2H5)B3N3H5 with HCl, followed by reduction of the 2-RB(3)N(3)H(5) . 3HCl intermediates with NaBH4. A structural determination of 2-(PhC(2)H(4))B3N3H11 confirmed that the B3N3 ring adopts a chair conformation with the alkyl group attached to B2 in an equatorial position.