A method for processing a nucleic acid, in which the nucleic acid is exposed to an aqueous medium which includes a polyol in sufficient proportion for at least a portion of the nucleic acid to enter or remain in an extra-solution phase. Thus, a polyol may be used to bind a nucleic acid which is in solution to a solid support or to wash a nucleic acid on a solid support while maintaining it on the support. The polyol may for example be a C2-C10 alkanediol.