The ribonucleoside-derivs. 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-derivs. may be suitably protected on the nucleobase and on the oxygen in 5'-position also. The new protection-groups in 2'-O-position are superior to conventional such protection the groups as they are not subject to isomerization and give higher coupling yields. Ribonucleoside I whereby R1 is a base of the purine- or pyrimidine-family or a deriv. of such a base, R2 is a proton or a substituted deriv. of phosphonic acid, R3 is a proton or a suitable protection-group, R4, R5, R6 are advantageously three identical or different alkyl- or aryl-substituents which together comprise between 6 and 30 carbons atoms. were prepd. and used in the synthesis of RNA. Thus, nucleoside phosphoramidite II was prepd. and used in the synthesis of RNA. [on SciFinder (R)]