The relaxation behaviour of a thermotropic polymer liquid crystal, poly(tetramethylene terephthaloyl-bis-4-oxybenzoate), and its derivatives with substitution in the mesogen and/or in the spacer, have been analysed by dynamic mechanical analysis and differential scanning calorimetry (d.s.c.). By comparing the relaxation spectra of the four polymers some conclusions can be drawn. They all show a β relaxation at low temperature and an α relaxation above room temperature. The β relaxation has been associated with local motions of the mesogenic unit. The substitution of chlorine in the mesogen increases the temperature of this relaxation by 60°C, but it is not affected by substitution in the spacer. The α relaxation has been associated with the glass transition of the amorphous phase. It increases by approximately 15°C with the introduction of lateral methyls in 1,4 position in the spacer, independently of substitution in the mesogen. The presence of crystalline order, besides the nematic mesophase, broadens this relaxation.