A recent study indicated that the water-saturated ionic liquid (IL) trihexyl(tetradecyl)phosphonium chloride ([P6,6,6,14][Cl]) provided a viable electrolyte for a Mg-air battery. However, there is limited literature on the properties of IL-water mixtures as battery electrolytes. The physical properties of [P6,6,6,14][Cl] were studied with the addition of both water and metal salts (MgCl2 and LiCl) using conductivity and self-diffusion coefficient measurements. The conductivity of the samples at low water concentrations is surprisingly enhanced by the addition of the metal salt, contrary to lithium IL electrolytes. It was also found that the conductivity of the IL was increased by an order of magnitude by saturation with water. NMR diffusion measurements were used to probe the behaviour of both the cation and the water in the mixtures. It was found that the addition of metal salts to the water-saturated [P6,6,6,14][Cl] did not affect the transport properties of the water or cation.