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Ionic liquid ion sources (ILIS) have been proposed as a source of alternative ion chemistries for surface etching, focused ion beams, and secondary ion mass spectrometry. These ion sources produce polydisperse ion beams containing several solvated ion species with a distribution of energies, and it is necessary to filter the ion beam to obtain monoenergetic beams with a single ion chemistry as required by nanomanufacturing and analytical applications. In this work, a Wien filter has been designed, built, and tested for use with an ILIS using the liquid 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, or EMI-FAP. comsol multiphysics simulations have been used to validate the design. The full and unfiltered ion beams have been characterized using time-of-flight mass spectrometry and retarding potential analysis, and these measurements confirm that the filter is effective in isolating monomer ions from heavier ion species in the beam. [ABSTRACT FROM AUTHOR]