The first results of a study aiming at an efficient prepn. of a large variety of 2'-O-[(triisopropylsilyl)oxy]methyl(=tom)-protected ribonucleoside phosphoramidite building blocks contg. modified nucleobases are reported. All of the here presented nucleosides have already been incorporated into RNA sequences by several other groups, employing 2'-O-tbdms- or 2'-O-tom-protected phosphoramidite building blocks (tbdms=(tert-butyl)dimethyl-silyl). We now optimized existing reactions, developed some new and shorter synthetic strategies, and sometimes introduced other nucleobase-protecting groups. The 2'-O-tom, 5'-O-(dimethoxytrityl)-protected ribonucleosides N2-acetyl-iso-cytidine, O2-(diphenyl-carbamoyl)-N6-isobutyryl-isoguanosine, N6-isobutyryl-N2-(methoxy-acetyl)purine-2,6-diamine ribonucleoside (=N8-isobutyryl-2-[(methoxyacetyl)amino]adenosine), 5-methyluridine, and 5,6-dihydro-uridine were prepd. by first introducing the nucleobase protecting groups and the dimethoxytrityl group, resp., followed by the 2'-O-tom group. The other presented 2'-O-tom, 5'-O-(dimethoxytrityl)-protected ribonucleosides inosine, 1-methyl-inosine 18, N6-iso-pent-2-enyl-adenosine, N6-methyl-adenosine, N6,N6-dimethyl-adenosine, 1-methyl-guanosine, N2-methylguanosine, N2,N2-dimethyl-guanosine, N6-(chloroacetyl)-1-methyladenosine, N6-(1S,2R)-2-[(tert-butyl)dimethyl-silyl]oxy-1-[2-(4-nitrophenyl)ethoxy]carbonylpropylamino carbonyl-adenosine (derived from L-threonine) and N4-acetyl-5-methyl-cytidine were prepd. by nucleobase transformation reactions from std., already 2'-O-tom-protected. Finally, all these nucleosides were transformed into the corresponding phosphoramidites, which are fully compatible with the assembly and deprotection conditions for std. RNA synthesis based on 2'-O-tom-protected monomeric building blocks. [on SciFinder (R)]