A series of new 2'-5'-oligonucleotide trimers carrying a 9-(2',3'-anhydro-beta-D-ribofuranosyl)-(59), 9-(3'-deoxy-beta-D-glycero-pent-3-enofuranosyl)-63, 9-(3'-azido-3'-deoxy-beta-D-xylofuranosyl)-(62), and 9-(3'-halo-3'-deoxy-beta-D-xylofuranosyl)adenine(60 and 61) moiety at the 2'-terminal end have been synthesized via the phosphotriester method. The properly protected, modified monomeric building blocks (6, 9, 16, 19, 27, 33, 36, 37, and 43) were obtained, in general, by a sequence of reactions, introducing the protecting groups into the right positions. Their condensations with the intermediary dimeric 2'-terminal phosphodiesters 48 and 49 led to the fully protected 2'-5'-trimers 50-58 which were deblocked to form the free 2'-5'-trimers 59-63. Easy elimination of HBr on deprotection did not allow to form the trimeric (3'-bromo-3'-deoxy-beta-D-xylofuranosyl)adenine analogue but only 63 carrying an unsaturated sugar moiety instead. The newly synthesized compounds have been characterized by UV and NMR spectra as well as by elemental analysis.