Abstract: A composition suitable for disinfecting a water supply consists essentially of an aqueous solution of monochloroamine and hydrogen peroxide. The weight ratio of the hydrogen peroxide to monochloroamine is from 2:1 to 50:1, preferably 4:1 to 20:1.An advantage of employing a combination of monochloroamine and hydrogen peroxide as disinfectant is that a comparatively long lasting residual bactericide is thereby provided.