Me4PF was investigated in the solid state, in the gas phase, and in solutions. Vibrational spectra of the solid and a single-crystal structure show an ionic tetramethylphosphonium fluoride. The compound crystallizes in the space group Pbca with a = 1016.0(1), b = 1018.0(1), c = 1205.8(4) pm, and Z = 8. The fluoride ion is nearly trigonal planar surrounded by three Me4P+ cations forming six H...F contacts between 218 and 240 pm. The compound is stable below 120 degreesC and sublimes in a vacuum. It possesses a phosphorane structure in the gas phase that was studied by electron diffraction and vibrational spectra, and additionally by theoretical calculations. The Me4PF molecule has a trigonal bipyramidal structure with one methyl group and the fluorine atom in axial positions and bond lengths of d(PCeq) = 182.6(4) pm, d(PCax) = 188.4(8) pm, and d(PF) = 175.3(6) pm. The compound is remarkably soluble in acetonitrile, water, and alcohols, and slightly soluble in benzene, dimethyl ether, and diethyl ether. The solutions were studied by H-1, C-13, F-19, and P-31 NMR spectroscopy. The hygroscopic Me4PF forms a tetrahydrate which crytallizes in the space group I4(1)/a with a = 1106.1(1) pm, c = 816.3(1) pm, and Z = 4. The fluoride ion in Me4PF.4 H2O is surrounded by four water molecules. These units form a three-dimensional network in which the Me4P+ cations are embedded without any contacts.