The effects of HF content on the ionic conductivity and the viscosity of low temperature molten fluorides, (CH3)4NF-·mHF, (CH3)3N-·mHF, KF-·mHF HF, and NH 4F-·mHF melts, were investigated. The ionic conductivity of the (CH3)3N-·mHF and (CH3) 4NF-·mHF melts increased, whereas their viscosity decreased with increasing the m value. The ionic conductivity and the viscosity of the (CH3)3N-·mHF melts were lower and higher, respectively, than those of the (CH3)4NF-·mHF melts at a given m value and a given temperature. The activation energy for ionic migration decreased with increasing the m value in both systems. Similar effects of the m value on the conductivity and viscosity were obtained for the KF-·mHF and NH4F-·mHF melts. The KF-·mHF melts exhibited lower conductivity and higher viscosity than the NH 4F-·mHF melts at a given m value and a given temperature. It was concluded that these effects of the HF content can be explained in terms of the concentration change of molecular HF and the resulting change in electrostatic cation-anion interactions.