A biocidal preparation comprises a biocidally effective amount of a mixture of a compound of the general formula: WHEREIN R1 is alkyl with eight to 18 carbon atoms or benzyl which is chlorinated or brominated in 2- and/or 4-position and of a compound of the general formula: WHEREIN R2 is alkyl with seven to 17 carbon atoms. The weight ratio of the compounds I and II in the mixture is about 2 : 1 to 1 : 5.