A method for producing a stable microbicidal composition. The method comprises steps of: (a) combining a glycol solvent of formula R(OCH2CHR1)nH, wherein R is C1-C6 alkyl and n has a number average value from 2 to 5 and R1 is hydrogen or methyl; dodecylbenzenesulfonic acid in an amount from 0.3 to 1.8 wt % based on the entire microbicidal composition; and a copper (II) salt selected from the group consisting of copper (II) hydroxide, copper (II) carbonate and mixtures thereof to form an initial mixture; wherein a mole ratio of copper:dodecylbenzenesulfonic acid is from 1:0.25 to 1:1.95; (b) maintaining the initial mixture at 40 to 80° C. until an amount of copper ion in solution is stable; and (c) adding 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one to produce a final mixture.