The ionic liquid (IL) trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl phosphinate ([P6,6,6,14][(iC8)2PO2]) has been shown to passivate magnesium alloy AZ31 at anodic potentials within the IL environment. In this study, [P6,6,6,14][(iC8)2PO2] IL was diluted in organic solvent acetonitrile to concentrations of 0.1 and 0.01M. Cyclic voltammetry (CV) at anodic potentials from open circuit potential (OCP) appeared to passivate the surface in both solutions to the same degree. Thicknesses of 80 and 29nm were determined for films formed under anodising conditions in neat IL and 0.1M IL in acetonitrile respectively. The results indicate that despite dilution in acetonitrile the electrochemical behaviour of the IL is essentially unchanged. However, the effect of dilution changes the process of film formation during anodising to 18V, resulting in different film thicknesses.