Two analogous series of thermotropic main-chain polyesters, poly(alkyl-4,4'-diphenoxy terephthalate)s and poly(alkyl terephthaloyl-bis-4-oxybenzoate)s, have been analyzed by dielectric techniques. Analysis of the dielectric properties as a function of temperature and frequency show three main processes, termed alpha, beta and beta*. The alpha transition has been associated with the glass transition of the amorphous phase. The low-temperature processes beta and beta* have been related to the local motion of internal and external polar groups in the mesogenic unit. The activation energy and dielectric strength values associated with these relaxations have been determined, and a very important effect of the crystalline order on these parameters has been observed. The effect of the thermal history on the dielectric behaviour has also been studied.