The analogous series ofp,p'-di-n-alkyl-substituted azoxy- and azobenzenes are synthesized. These compounds show low melting points and nematic or smectic behaviour. The melting points are generally lower for the azoxy compounds than for the analogous azo compounds. A probable explanation is the non-planar conformation of the azoxy compounds as compared with the planar conformation of the azo compounds. The influence of a permanent dipole moment in the azoxy series and the absence of a permanent dipole moment in the azo series is investigated. The clearing point is increased by the presence of a permanent dipole moment. The dielectric constants parallel and perpendicular to the molecular axis are measured for some typical members of the series. For both series we find ε‖> ε⊥, the difference being greater for the azo series than for the azoxy series.