Provided is a preparative method for isolating RNA comprising an oligo-or polynucleotide from a sample, which method comprises: (a) treating the sample with a reactant capable of covalently modifying the 2'-OH position of the ribose rings of the RNA under conditions so that a proportion of the 2'-OH positions of the ribose rings bear a substituent; and (b) preparing isolated RNA therefrom by separating material containing the substituent from the sample on the basis of a property of the substituent.