Ionic and nonionic species in (CH3)(4)NF(.)mHF melts (in = 3.0-5.0) were investigated by H-1 NMR, IR spectroscopy, Raman spectroscopy, and high-energy X-ray diffraction measurements. Three types of anions, (FH)F-. (FH)(2)F-, and (FH)(3)F-, were identified in these melts. The (FH)(2)F- anion was thermodynamically the most stable of the complex anions in the (CH3)(4)NF(.)mHF (3.0 less than or equal to m less than or equal to 5.0), and its concentration was the highest. (FH),F- anions with x = 4 and 5 were not present at room temperature though the presence of (FH)(x)F- (x = 1-5) has been known in the solid-state crystals. In the (CH3)(4)NF(.)mHF melts with m = 3.0-5.0, the number of HF combined with F- is three or less, and excess HF exists as molecular HF.