© The Electrochemical Society. The conductivity of vanadium flow battery (VFB) catholytes was measured over a range of vanadium concentrations [V], sulfur concentrations [S], states of charge (SoC) and temperatures. In general, conductivity variations are highly linear. In particular, the linear variation of conductivity with SoC suggests that the conductivity of a VFB catholyte can be empirically modelled to a high degree of accuracy as the simple sum of the constituent VIVand VVsolution conductivities. A model of conductivity variation as a function of [V] and [S] was developed for both VIVand VVsolutions. A model of VFB catholyte conductivity as a function of [V], [S], SoC and temperature was then developed by simply adding the individual conductivities of the component solutions. This empirical model was accurate to within <1%. Such a model is necessary for the long term use of conductivity as a measure SoC, as both [V] and [S] vary as the cell is cycled.