A biocide composition comprises an effective amount of a biocide and an effective amount of a biocide activator selected from the group consisting of an alkyl phosphate, an alkenyl phosphate, a hydroxyalkyl phosphate, a polyoxyalkylene alkyl ether phosphate, a salt thereof, a polyoxyalkylene alkenyl ether phosphate, a salt thereof, a polyoxyalkylene hydroxyalkyl ether phosphate and a salt thereof.