Selective acylation of ribonucleotides and ribonucleosides can be achieved by using N-acylimidazole on a preparative scale with good yields (50-80%). For uridine 3’-phosphate (Up): inthe presence of MDCAI, the 2’-O-acyl-derivative is the main product, while in the presence of an excessof TEAH, the 5’-O-acyl-derivative is the main product. For ribonucleosides (U_R or A_R or Ψ_R):in the presence of MDCAI, the acylations take place preferably at 2’-OH or 3’-OH of ribonucleosidesand only 3’-O-acyl-derivatives can be isolated by crystallization ; in the presence of an excess of TEAH,5’-O-acyl-derivative is obtained as the main product. Arabinonucleoside and deoxyribonucleosideare only slowly acylated to form 5’-O-acyl-derivatives as the main products by acylimidazole in thepresence of MDCAI. Possible mechanisms of these acylations have been discussed. Selective acylation of ribonucleotides and ribonucleosides can be achieved by using N-acylimidazole on a preparative scale with good yields (50-80%). For uridine 3’-phosphate (Up): inthe presence of MDCAI, the 2’-O-acyl The ribonucleosides (U_R or A_R or Ψ_R): in the presence of MDCAI, the acylations take place preferably at 2’-OH or 3’-OH of ribonucleosides and only 3’-O-acyl-derivatives can be isolated by crystallization; in the presence of an excess of TEAH, 5’-O-acyl-derivative is obtained as the main product. Arabinonucleoside and deoxyribonucleoside only slowly acylated to form 5’-O-acyl-derivatives as the main products by acylimidazole in thepresence of MDCAI. Possible mechanisms of these acylations have been discussed.