The inclusion of NH4+ as counterions in Car-Parrinello molecular dynamics (CPMD) simulations of anionic uranyl(VI) complexes is proposed as a viable approach to modeling "real" aqueous solutions. For [UO2F4(H2O)](2-) in water, it is shown that the inclusion of two NH4+ ions strengthens the bond between uranyl and the water ligand by ca. 2 kcal/mol, improving the accordance with experiment. According to CPMD simulations for [UO2X5]-[NH4](3) (X = F, OH) in water, the fifth fluoride is bound much stronger than the fifth OH-. Implications for a recently proposed model for oxygen exchange in uranyl hydroxide are discussed.