2′-Deoxycytidine-, 2′-deoxyadenosine- and 2′-deoxyguanosine-5′-O-(4,4′-dimethoxytriphenylmethyl)-3′-O-(2-chlorophenyl-2-cyanoethyl)phosphate have been anchored to a solid support (polydimethylacrylamide), through the exocyclic amino group. 2′-Deoxythymidine-5′-O-(4,4′-dimethoxytriphenylmethyl)-3′-O-(2-chlororophenyl-2-cyanoethyl)phosphate has been attached as well, by means of the N3 and/or O4 positions of the base moiety. The resulting polymer, which permits chain elongation from either end, can be conveniently used for the solid phase synthesis of cyclic oligodeoxyribonucleotides. The preparation of a number of such molecules containing up to 10 base residues by the phosphotriester approach is described