The [(triisopropylsilyl)oxy]methyl (TOM) group is a useful protecting group for the 2'-OH of ribonucleosides to be used for oligoribonucleotide synthesis by the phosphoramidite method. It is completely stable to all reaction conditions applied during assembly and the first deprotection step. It does not interfere with the coupling reaction and leads to very good coupling yields under DNA-coupling conditions. The final cleavage occurs quantitatively without concomitant destruction of the RNA product. This unit describes the synthesis of TOM-phosphoramidites from 2'-O-TOM-5'-O-dimethoxytrityl-N-acetyl ribonucleosides, oligoribonucleotide assembly on an automated DNA synthesizer, and subsequent deprotection. Preparation of the TOM-protected ribonucleosides is presented elsewhere in the series.