Both SeO2 and Se(:NCMe3)2 react with Et3B by 1,2-ethyloboration. In the case of SeO2, ethane, ethene, Et2SeH2, (Et2B)2O, (EtBO)3 and compd. I are formed under heating to 65 Deg. Compd. I is also formed in the reaction of SeO2 with (Et2B)2O. Treatment of H2SeO3 with Et3B or (Et2B)2O provides further routes to I. The reaction of Se(:NCMe3)2 with Et3B starts already <-50 Deg. The ethaneselenic acid deriv. II is formed which begins to decomp. at -50 Deg with elimination of ethene to give finally Et2BNHCMe3, (Et2B)2NCMe3, and Et2Se. Transborylation of I with (9-BBN)2O affords compd. III which crystallizes in the monoclinic space group P21/n with a = 667.1, b = 1282.5, c = 1289.1 pm and b = 93.06 Deg (118 K). All reactions were monitored by 11B- and 77Se NMR. The reactions of SeO2 with Et3B and (Et2B)2O and the transborylations were studied by 17O NMR with 17O-enriched compds.