The hydrophobic hydrophilic character of a series of 15 tetra-alkylphosphonium ionic liquids with aprotic heterocyclic anions (AFIAs) was studied by measuring their liquid-liquid equilibrium with water. Specifically, the solubility of water in the ionic-liquid-rich phase was determined at room temperature by Karl Fischer titration. The effect of the tetra-alkyl-phosphonium cation was studied by varying the alkyl chain length; as expected, hydrophobicity increases with increasing alkyl chain length. The effect of the anion was studied by pairing the tetra-alkyl-phosphonium cation with different AHAs and by adding different substituents on the anion. The anion nature, basicity, and electron withdrawing effect of different substituents proved to have a significant impact on the IL's hydrophobicity. Changes in the alkyl chain length of the cation and the nature of the anion are able to tune the solubility of water in the ILs over an order of magnitude.