CMIT/MIT is a liquid biocide with outstanding advantages such as rapid inhabitation of growth and macromolecule synthesis, broad spectrum activity, effective at low concentration and against biofilm as well as a wide pH range, biodegradable non-persistent in the environment, non-surface activity, infrequent dosing, easily deactivated, compatibility, low toxicity, water soluble, non-foaming, cost effectiveness etc. The active ingredient is a mixture of two isothiazolines. CMIT/MIT is ideally suited to meet the requirement of an industrial water treatment biocide for bacteria, fungi and algae paper mill slimicide.