Lanthanide (III) ions have shown enormous catalyses for the hydrolysis of the phosphodiester linkages in RNA, indicating their high potential for versatile applications to biotechnology and molecular biology. The activity monotonically increases with increasing atomic number in the lanthanide series, the last three ions (Tm³⁺, Yb³⁺, and Lu³⁺) being the most active. Non-lanthanide metal ions are virtually inactive. The pseudo first-order rate constant for the hydrolysis of adenylyl (3'-5') adenosine (ApA) by LuCl₃ (5 mmol•dm^-3) at pH 7.2 and 30°C is 1.9×10^-1min^-1 (the half-life is only 3.6min), corresponding to 10⁸-fold acceleration. The product is an equimolar mixture of adenosine and its 2'- or 3'-monophosphate without any byproducts. The 2', 3'-cyclic monophosphate of adenosine is not accumulated much in the reaction mixture. Lanthanide ions also efficiently hydrolyze oligoribonucleotides without a specific base-preference. In ApA hydrolysis by NdCl₃ and GdCl₃, the dependence of the hydrolysis rate on either the pH or concentration of the metal salt coincides fairly well with the corresponding profile of the equilibrium concentration of the bimetallic hydroxo-cluster [M₂(OH)₂]⁴⁺ (M=metal ion). Both the formation of the pentacoordinated intermediate and its decomposition are greatly promoted by lanthanide ions. A catalytic mechanism in which two metal ions (or their coordination water) in these tetracationic hydroxo-clusters show acid/base cooperation is proposed.