The conditions for initiation of EHD instability at frequencies of 50–150 Hz and for DC for eight nematic materials having a positive dielectric anisotropy (λϵ > 3) have been experimentally studied. The dependences of the threshold voltage for the EHD instability on electrical conductivity, as well as on the translational and rotational viscosities of the LC materials have been found. For the measurements substances with strongly differing (from 10⁻⁹ to 5 × 10⁻⁷ Ω⁻¹ m⁻¹) electrical conductivities and profound temperature dependence of the viscosities were chosen. Analysis of the results based on the isotropic model of EHD instability shows good agreement between theory and experiment.