Electro-optical properties of nematics of the trimer series 4,4-bis-[omega-(4-cyanobiphenyl-4yloxy)alkyloxy]biphenyls have been investigated by the method of orientational elastic deformations in electric fields. The number n of CH2 groups in the oxyalkyl fragment was varied from n 2 to n 10. A strong odd-even effect was observed in the optical anisotropy Deltan and in the values of the threshold potential V0 for the trimer nematics. These effects are, respectively, caused by oscillations of the degree of inter- and intra-molecular orientational order when n is varied. The great difference in the dependences of Deltan and V0 on n is discussed and compared with data for dimers.