Compounds with trihexyl(tetradecyl)phosphonium (THTP) cations and first row transition metal-substituted Keggin polyoxoanions were synthesised, having the general formula (THTP)_m[PW₁₁O₃₉M(H₂O)] with M = Fe, Co, Mn and m = anion charge. All compounds were characterized by spectroscopic and analytical techniques. These studies indicated that both the polyoxometalates and the cations are present in the new compounds without major alterations. All compounds are coloured liquids at room temperature and are stable up to ca. 300 °C. Studies conducted at low temperatures point to a solidification process at about −25 °C for the Fe and Co materials. Electrochemical measurements were performed for (THTP)₄[PW₁₁O₃₉Fe(H₂O)] on a functionalized glassy carbon electrode. This work was complemented by the preparation and characterization of compounds with the anions [PW₁₁O₃₉]⁷⁻ and [P₂W₂₀O₇₀(H₂O)₂]¹⁰⁻, which were also liquids at room temperature.