The invention relates to the use of polymeric guanidinium hydroxides based on a diamine which contains oxyalkylene chains and/or alkylene groups between two amino groups, obtainable by polycondensation of a guanidine acid addition salt with the diamine, whereby a polycondensation product in the form of a salt is obtained, which subsequently is converted into the hydroxide form via basic anion exchange, for controlling microorganisms.