Selectively deuterated liquid crystalline copolyesters of 4,4’-dihydroxy, alpha, alpha’-dimethylbenzalazine were investigated by proton and deuterium n.m.r. N.m.r. spectra were recorded over a wide temperature range : from 100 degrees C (50 to 60 degrees C below the solid to nematic phase transition, T-m) to 255 degrees C which is well above the isotropic-nematic phase transition. The main-chain liquid crystalline polymer does not align in the n.m.r. magnetic field below T-m + 20 degrees C, where T-m represents the melting temperature measured by differential scanning calorimetry. However, from the splitting Delta nu between the peaks in the powder pattern we conclude that the sample is composed of randomly oriented domains. Within these domains the same type of molecular motion occurs as in the monodomain nematic phase at T > T-m + 20 degrees C. Above T-m + 20 degrees C the H-2 n.m.r. spectra show the existence of doublets which are characteristic of an aligned monodomain nematic phase. High degrees of orientational order, S-zz, measured by both proton and deuterium n.m.r. are practically independent of sample thermal history. Values of S-zz at the nematic to isotropic phase transition and the order parameters of the flexible units are consistent with data reported for other polymers of similar chemical structure.