The present invention provides a new labeling reagent for preparing modified oligonucleotides and processes for their production wherein these oligonucleotides contain at least once the structure P.dbd.N--SO.sub.2-benzole-L-M-X, characterized in that L is either --(CH.sub.2)n- or polyethylene glycol, M is selected from a group consisting of --NH--, --O--, --S--, and --COO--, and X is either a protecting group or a detectable unit. L is preferably either --(CH.sub.2)n- or polyethylene glycol.