The use of alkyl-3-methylimidazolium alkylsulfonate ionic liquids for implementing aqueous biphasic systems is studied in this work for the first time. The ability of high charge density inorganic salts, such as K3PO4, to promote phase segregation in aqueous solutions containing the ionic liquids 1,3-dimethylimidazolium methylsulfonate ([C(1)mim][C1SO3]), 1-ethyl-3-methylimidazolium hexylsulfonate ([C(2)mim][C6SO3]), 1-ethyl-3-methylimidazolium butylsulfonate ([C(2)mim][C4SO3]), 1-butyl-3-methylimidazolium methylsulfonate ([C(4)mim][C1SO3]), 1-butyl-3-methylimidazolium ethylsulfonate ([C(4)mim][C2SO3]), 1-pentyl-3-methylimidazolium methylsulfonate ([C(5)mim][C1SO3]), 1-hexyl-3-methylimidazolium methylsulfonate ([C(6)mim][C1SO3]) and 1-hexyl-3-methylimidazolium ethylsulfonate ([C(6)mim][C2SO3]) was experimentally determined at 298.15 K and atmospheric pressure. In general, the hydrophobicity of the ionic liquids studied is affected by the increase of the alkyl chain length. However, the position of the alkyl chain, whether in the cation or in the anion affects in a different way the lipophilic effect of the ionic liquid. Two ionic liquids with the same number of carbon atoms, the one with a longer chain in the anion is the more hydrophobic.