Abstract: The ribonucleoside-derivatives serve for the synthesis of ribonucleic acids and comprise a triple substituted silyloxymethyl-group as a protection-group on the oxygen atom in 2'-position. The ribonucleoside-derivatives may be suitably protected on the nucleo-base and on the oxygen in 5'-position also. The new protection-groups in 2'-O-position are superior to conventional such protection-groups as they are not subject to isomerization and give higher coupling yields. The general formula of the ribonucleoside-derivative is: ##STR1## whereby R.sup.1 is a base of the purine- or pyrimidine-family or a derivative of such a base,R.sup.2 is a proton or a substituted derivative of phosphonic acid,R.sup.3 is a proton or a suitable protection-group,R.sup.4, R.sup.5, R.sup.6 are advantageously three identical or different alkyl- or aryl-substituents which together comprise between 6 and 30 carbon atoms.