Vanadium(v) oxo complexes with tetradentate diamine bis(phenolato) "salan" ligands of the type LVO(OiPr) (L is salan) with different steric and electronic substitutions at the ortho and para positions to the binding phenolato moiety were synthesized and their hydrolytic stability and cytotoxicity were analyzed. With one exception bearing large steric groups, all complexes examined displayed marked cytotoxic activity, comparable to, and often higher than, that of cisplatin. While the hydrolytic stability changed significantly depending on the substituent at the ortho position relative the O-donor with little effect of para substitutions, the cytotoxic activity largely was not affected, and high cytotoxicity was recorded also for relatively unstable complexes. Additional measurements revealed that the cytotoxicity is largely maintained following pre-incubation of up to 18 hours of the complexes in the biological medium prior to cell addition, suggesting that hydrolysis products might serve as the active species. In addition, appreciable cytotoxic activity was measured for an isolated hydrolysis product that was analyzed crystallographically to exhibit a dimeric structure with bridging oxo ligand where both metal centers are bound to the salan ligand, supporting the aforementioned conclusions.