The dielectric behaviors of typical benzene monosubstitutes, bromobenzene (Br-Bz) and benzonitrile (NC-Bz), were investigated up to 3 THz in the pure liquid state over a temperature range from 10 to 60 degrees C to understand differences in molecular motions of these simple, planar molecules bearing rather different electric dipole moments: 1.72 and 4.48 D for Br-Bz and NC-Bz in gaseous state, respectively. Temperature dependence of spin-lattice relaxation time (T-1) for C-13 NMR and viscosities for these liquids were also determined to obtain information for molecular motions. Moreover, depolarized Rayleigh scattering (DRS) experiments were carried out for both liquids at 20 degrees C to determine frequency dependencies of optical susceptibilities up to 8 THz directly relating to rotational motions of their molecular planes. Most Br-Bz molecules rotate freely over a temperature range examined, showing a Kirkwood correlation factor close to g(K) similar to 1.0 at dielectric Debye-type relaxation times (ca. 18 ps at 20 degrees C) essentially identical to microscopic (dielectric) relaxation times evaluated from T-1 C-13 NMR data. A small amount of Br-Bz molecules forms dimeric intermolecular associations in an antiparallel configuration of dipole moments. On the other hand, NC-Bz molecules form stable dimers in the antiparallel dipole configuration at a population much higher than that of Br-Bz because of a markedly greater dipole moment than that of Br-Bz. A major dielectric relaxation mechanism for NC-Bz found at ca. 70 ps at 20 degrees C results from the dissociation process of dimers with a lifetime longer than a rotational relaxation time, observable as a minor dielectric relaxation mechanism at ca. 12 ps at 20 degrees C, of individual monomeric NC-Bz molecules without the formation of dimers. The formation of stable dimers in an antiparallel configuration is responsible for the observed small g(K) values, ca. 0.5, and disagreement between major (or minor) dielectric relaxation times and microscopic dielectric relaxation times over the entire temperature range examined.