Dielectrophoresis (DEP) has been widely studied for its potential as a biomarker-free method of sorting and characterizing cells based upon their dielectric properties. Most studies have employed voltage signals from ~1 kHz to no higher than ~30 MHz. Within this range a transition from negative to positive DEP can be observed at the cross-over frequency fx01. The value of fx01 is determined by the conductivity of the suspending medium, as well as the size and shape of the cell and the dielectric properties (capacitance, conductivity) of its plasma membrane. In this work DEP measurements were performed up to 400 MHz, where the transition from positive to negative DEP can be observed at a higher cross-over frequency fx02. SP2/O murine myeloma cells were suspended in buffer media of different osmolarities and measurements taken of cell volume, fx01 and fx02. Potassium-binding benzofuran isophthalate (PBFI), a potassium-sensitive fluorophore, and flow cytometry was employed to monitor relative changes in intracellular potassium concentration. In agreement with theory, it was found that fx02 is independent of the cell parameters that control fx01 and is predominantly determined by intracellular conductivity. In particular, the value of fx02 is highly correlated to that of the intracellular potassium concentration.