Abstract: The present invention relates to stabilized compositions, or compositions that are stable when stored, comprising: (A) 2.5 to 15% by weight of the isothiazolinone 2-methyl-2H-isothiazolin-3-one and 1,2-benzisothiazolin-one and/or its sodium and/or potassium salt, (B) 0.5 to 50 ppm by weight of copper(II) ions, in relation to the weight fraction of component (A), (C) 100 to 2,500 ppm by weight, in relation to the total composition, of at least one oxidizing agent selected from the group consisting of sodium iodate, potassium iodate, sodium bromate and potassium bromate, or mixtures of these and (D) water. The invention further relates to a process for the preservation of technical materials by means of the aforementioned compositions, as well as the technical materials treated therewith.