A new carbon paste electrode is described, which contains the room temperature ionic liquid (RTIL) tri(tert-butyl)(dodecyl)phosphonium tetrafluoroborate as binder. The advantages of this electrode are a high conductivity, very wide electrochemical window (5.6 V from 2.7 to −2.9 V, one of the widest ever reported for RTILs), stability in time, and reproducibility. This RTIL-carbon paste electrode (CPE) allows determining the current-voltage characteristics of redox-active compounds. Thus, the newly synthesized insoluble compound poly-tris(μ²-1,1′-ferrocenediyl-phenylhydrophosphinato-phenylphosphinato)-iron(III) tetrahydrofuran solvate {μ²-[Fe^II(η⁵-C₅H₄–P(PhOO)(η⁵-C₅H₄–P(PhOOH))]₃Fe^III}·THF was studied, and a quasi-reversible three-electron oxidation could be observed at a potential more positive than that of ferrocene. A comparison of voltammograms on the paraffin-CPE and on the novel RTIL-CPE shows the advantages of the latter.