This paper describes molecular dynamics (MD) simulations of three ionic liquid (IL)/water mixtures at various water mole fractions: 1-butyl-3-methylimidazolium (BMIM+)/BF4-, 1-octyl-3-methylimidazolium (OMIM+)/BF4-, and OMIM+/CI-. The goal is to elucidate the effect of alkyl side chain length and anion on the structure and dynamics of the mixtures. Replacing the BMIM+ cation with OMIM+ results in stronger aggregation of the cations and slower diffusion of the anions. Replacing the BF4- anion with Cl- alters the water distribution at low water mole fractions and slows diffusion in the mixtures. Potential experimental manifestations of the behavior in both cases are given. (C) 2010 Elsevier B.V. All rights reserved.