In this work, Pa(V) monocations have been studied in liquid water by means of density functinal theory (DFT) based molecular dynamic simulations (CPMD) and compared with their U(VI) isoelectronic counterparts to understand the peculiar chemical behavior of Pa(V) in aqueous solution. Four different Pa(V) monocationic isomers appear to be stable in liquid water from our simulations: [PaO2(H2O)s](+)(aq), [Pa(OH)(4)(H2O)(2)](+)(aq), [PaO(OH)(2)(H2O)(4)](+)(aq), and [Pa(OH)(4)(H2O)(3)](+)(aq). On the other hand, in the case of U(VI) only the uranyl, [UO2(H2O)(5)](2)(+)(aq), is stable. The other species containing hydroxyl groups replacing one or two oxo bonds are readily converted to uranyl. The Pa-OH bond is stable, while it is suddenly broken in U-OH. This makes possible the formation of a broad variety of Pa(V) species in water and participates to its unique chemical behavior in aqueous solution. Further, the two actinyl oxocations in water are different in the ability of the oxygen atoms to form stable and extended H-bond networks for Pa(V) contrary to U(VI). In particular, protactinyl is found to have between 2 and 3 hydrogen bonds per oxygen atom while uranyl has between zero and one.