A novel composition has been discovered for decontaminating biofilm-contaminated surfaces (the “Biocidal Complex”). The novel composition both kills bacteria and destroys at least a part of the biofilm. The composition comprises an effective amount of a free-radical generating compound (e.g., hydrogen peroxide), a disinfectant from the “GRAS” list of food-safe compounds (e.g., thymol), and an acid sulfate such as sodium bisulfate (NaHSO4) to acidify the solution and help catalyze free radical formation. A preferred method of using this invention employs a multi-component approach that permits long-term storage of the components in a stable, concentrated form. Immediately before use, the components are mixed and then applied to the biofilm-contaminated surface. A separate metal catalyst for the generation of free-radicals may be added to increase the production of free radicals in the Biocidal Complex. This invention offers a safe, effective, and easy method for disinfecting and decontaminating biofilm-contaminated surfaces, including dental unit water lines.