This study examines the effect of structural components of an ionic liquid (IL), including the choice of the anion, alkyl chain length on the cation, and substitution on the cation, on the mutual solubility with water and with 1-octanol. This is important because ionic liquids have been shown to be good replacements for organic extraction solvents in some liquid separations. In addition, we use the measured data to predict 1-octanol/water partition coefficients for the ionic liquids using the nonrandom two liquid (NRTL) and electrolyte nonrandom two liquid (eNRTL) excess Gibbs energy models. Specifically, the mutual solubilities of 15 different imidazolium, pyridinium, and quaternary ammonium ionic liquids with water were measured at (23.5 +/- 1) degrees C. An increase in the alkyl chain length or the substitution decreases the mutual solubility with water. The [BF4](-) and [B(CN)(4)](-) anions are significantly more hydrophilic than [PF6](-), [(CF3SO2)(2)N](-), and [(CF3SO2)(3)C](-). The mutual solubilities of 10 ILs with 1-octanol were measured at (23.5 +/- 1) degrees C. For these systems, the longer the alkyl chain, the more soluble the IL was in the alcohol, while ring substitution has little effect. From this data, 1-octanol/water partition coefficients of 10 ionic liquids were predicted using the NRTL and eNRTL models.