Dielectric properties of several compounds with cyano and fluoro substituents were studied in the frequency range of 1 kHz - 3 GHz. Most of compounds exhibit nematic mesophase in a broad temperature range. The principal permittivity components and the relaxation processes were studied in the isotropic and the nematic phases. The dipole structure of molecules was analysed. The relaxation times and the activation barriers characterizing the principal molecular motions were determined. The substances show strong dependence of the dielectric anisotropy upon frequency: it changes its sign at high frequencies. Such compounds are useful to formulation of nematic mixtures for double frequency addressing system.