Oligonucleotide comprising at least one internucleotide phosphorus atom protected with a group of formula --X.sup.aSiR.sup.3R.sup.4R.sup.5 provided. X.sup.a represent 0 or S, and R.sup.3, R.sup.4 and R.sup.5 each independently are optionally substituted hydrocarbyl groups, selected such that that total number of carbon atoms in R.sup.3 plus R.sup.4 plus R.sup.5 is 4 or more. Process for the preparation of these oligonucleotides, intermediate compounds useful therein, and process for the preparation of the intermediate compounds are also provided.