Abstract: The method for the preparation of aqueous solutions containing performic acid formed by reaction of formic acid and hydrogen peroxide n the presence of a catalyst, the catalyst being a compound containing at least one ester group and/or group (a) differing from a carboxylic acid group and an alcoholic group, preferably a carboxylic acid ester, and aqueous solutions containing performic acid, formic acid, hydrogen peroxide, and a compound containing at least one ester group and/or group (a) differing from a carboxylic acid group, and the use of such solutions.