V(NAd)Me2(L) [2a, L = 2-ArNCH2(C5H4N), Ar = 2,6-Me2C6H3], prepared from V(NAd)Cl2(L) (1) by reaction with LiMe (2.0 equiv), exhibited remarkable catalytic activities for ethylene dimerization in the presence of MAO affording 1-butene with high selectivity [TOF = 1 120 000–1 530 000 h–1 (311–425 s–1), C4′ = 97.1–98.4%], and the catalyst performances (activity, selectivity) were similar to those by the dichloride analogue (1) under the same conditions. The dimethyl complex (2a) reacted with 1.0 equiv of R′OH to yield the mono alkoxide complexes, V(NAd)Me(OR′)(L) [R′ = OC(CF3)3 (3a), OC(CH3)(CF3)2 (3b), OC(CH3)3 (3c)], and structures of these complexes (3a–c) and 2a were determined by X-ray crystallography. Reactions of 2a with [Ph3C][B(C6F5)4] in Et2O and 3c with B(C6F5)3 in THF afforded the corresponding cationic complexes confirmed by NMR spectra. Both NMR and V K-edge XANES analysis of the toluene or toluene-d8 solution of 1 and 2a did not show any significant changes in the oxidation state upon addition of MAO, Me2AlCl, or Et2AlCl (10 equiv). Resonances ascribed to formation of the other vanadium(V) species were observed in the 51V NMR spectra, and no significant differences in the XANES spectra (V–K pre-edge peaks and edge) were observed from 1 or 2a upon addition of Al cocatalyst. Taking into account these results and others, it is thus suggested that cationic vanadium(V) alkyl/hydride species play a role in this catalysis.