A method for preparing an aqueous composition for sterilization, which comprises the step of mixing an acylimidazole represented by the general formula (II):wherein R represents a substituted or unsubstituted C1-C20 alkyl group, or a substituted or unsubstituted C6-C30 aryl group, with an aqueous solution containing a hydrogen peroxide source to produce a corresponding peroxycarboxylic acid. An aqueous composition for sterilization suitable for sterilization of medical apparatuses such as endoscopes, which can be safely and quickly prepared upon use, and a method for preparing the same are provided.
