Several poly(ester imide)s based on 4-N-(carboxyphenyl)trimellitimide, 4-N-(carboxyethenylphenyl)trimellitimide, 4-hydroxy-N-(4-hydroxyphenyl)phthalimide and long aliphatic spacers have been investigated by different solid-state NMR techniques. The conformations of the methylene units were studied by the g-gauche effect of the 13 C chemical shift. In the frozen smectic LC phase, an alternating sequence of trans-conformations and disordered segments is predominant. In contrast, the spacers in the smectic-crystalline phase are capable of forming ordered trans-trans conformations. The amount of tt-conformations is found to increase with the spacer length and depend on the packing of the mesogens and the type of linkage between mesogen and spacer. The thermal stability above 100ΠC and the segmental mobility of the ttconformations, as measured by 13 C/ 1 H wide line separation NMR, suggest a ropelike arrangement of the spacers. The tt-sequences are located in the outer parts rather than in the center of the spacer layer. Dephasing delay experiments on analogous polymers, which are deuteriated selectively in the four central methylene groups of the spacers, prove that these segments do not contain tt-conformations. Consequently, the ordering is due to the molecular constrains exerted by the rigid mesogenic groups and not by lateral van der Waals interactions between adjacent spacer segments. In a random copolymer with two different spacer lengths, the shorter spacer is found to be more extended than in the corresponding homopolymer.