To understand the ionic and non-ionic species in (CH3) 4NF·mHF, (CH3)3N·mHF, (C 2H5)4NF·mHF, and (C2H 5)3N·mHF melts, the structures of these melts were investigated by infrared spectroscopy, NMR and high-energy X-ray diffraction. IR spectra revealed that the complex fluoride anions such as (HF) nF- (n = 1, 2, 3) and molecular HF are present in all melts. Ionic conductivity and viscosity of these melts were measured and correlated with their cationic structure. The ionic conductivity of the R 4N+-systems was higher than those of corresponding R 3NH+-systems. (CH3)4N+ and (CH3)3NH+ cation gave higher ionic conductivity than (C2H5)4N+ and (C2H5)3NH+ cation, respectively. It was concluded that these effects on ionic conductivity can be explained by the cationic structure and the concentration of molecular HF in the melts. ©The Electrochemical Society.