We have studied pentafluoroaniline (C6F6NH2), pentachlorophenol (C6Cl5OH) and one solid solution of pentachlorophenol-hexachlorobenzene (C6Cl6) using dielectric spectroscopy (f < 2 MHz) and differential scanning calorimetry. The dielectric spectra in the disordered phase I of the first two samples did not conform to the Havriliak-Negami shape function; however, another process of smaller magnitude was noticed on the higher frequency side. The resulting relaxation rate of the dominant process along with the literature data suggests that the primary relaxation process in the rotationally disordered phase of hexa-substituted benzenes (HSBs) conforms to Arrhenius equation with apparent activation energy (E) in the range of 30-80 kJ/mol, dependent on molecular weight/size. The JG-relaxation of a number of HSBs in glassy o-terphenyl matrix was also been studied which allowed us to propose that the observed primary or a-relaxation in the neat HSBs is due to the hexad-rotation. Deviation of dielectric spectra from Debye behaviour is discussed. (C) 2014 Elsevier B.V. All rights reserved.