Phosphorylation of N-acyl-5'-O-DMTr-d-nucleosides, or similarly protected DNA-dimers having a free 3'-OH group, with 2-chlorophenyl-0,0-bis(1-benzotriazolyl)phosphate affords reactive 3'-phosphotriester derivatives. The latter intermediates can be used, without further purification, for the synthesis of DNA-fragments on the controlled pore glass/long chain alkylamine support. Further, 2-cyano-1,1-dimethylethoxy dichlorophosphine proved to be very suitable for the preparation of 5'-phosphorylated DNA-fragments on the same type of solid support.