The present application relates to biocidal substances comprising at least one isothiazolinone from the group consisting of 1,2-benzisothiazolin-3-one (BIT) and 2-methyl-4-isothiazolin-3-one (MIT), and at least one N-alkyl-guanidinium salt, methods for the production thereof, and their use for protecting technical materials and products which can be attacked by microorganisms.
