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═N—SO
2
-benzole-L-M-X, characterized in that L is either —(CH
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
2
)n- or polyethylene glycol.