Microhydration effects upon the adenine-uracil (AU) base pair and its radical anion have been investigated by explicitly considering various structures of their mono- and dihydrates at the B3LYP/DZP++ level of theory. For the neutral AU base pair, 5 structures were found for the monohydrate and 14 structures for the dihydrate. In the lowest-energy structures of the neutral mono- and dihydrates, one and two water molecules bind to the AU base pair through a cyclic hydrogen bond via the N-9-H and N-3 atoms of the adenine moiety, while the lowest-lying anionic mono- and dihydrates have a water molecule which is involved in noncyclic hydrogen bonding via the 04 atom of the uracil unit. Both the vertical detachment energy (VDE) and adiabatic electron affinity (AEA) of the AU base pair are predicted to increase upon hydration. While the VDE and AEA of the unhydrated AU pair are 0.96 and 0.40 eV, respectively, the corresponding predictions for the lowest-lying anionic dihydrates are 1.36 and 0.75 eV, respectively. Because uracil has a greater electron affinity than adenine, an excess electron attached to the AU base pair occupies the pi* orbital of the uracil moiety. When the uracil moiety participates in hydrogen bonding as a hydrogen bond acceptor (e.g., the N-6-H-6a center dot center dot center dot O-4 hydrogen bond between the adenine and uracil bases and the O-w-H-w center dot center dot center dot N and O-w-H-w center dot center dot center dot O hydrogen bonds between the AU pair and the water molecules), the transfer of the negative charge density from the uracil moiety to either the adenine or water molecules efficiently stabilizes the system. In addition, anionic structures which have C-H center dot center dot center dot O-w contacts are energetically more favorable than those with N-H center dot center dot center dot O-w hydrogen bonds, because the C-H center dot center dot center dot O-w contacts do not allow the unfavorable electron density donation from the water to the uracil moiety. This delocalization effect makes the energetic ordering for the anionic hydrates very different from that for the corresponding neutrals.