Abstract: A method of controlling the amount of conductivity increasing substances, e.g., free available halogen, in particular free available chlorine, present in an aqueous medium is described. In accordance with the method of the present invention, an aqueous medium is provided and its conductivity determined. A source of at least one conductivity increasing substance is introduced controllably into the aqueous medium, and a second conductivity measurement is made. The determined conductivities are compared, and in response to this comparison, the amount of conductivity increasing substance added to the aqueous medium is adjusted. In a preferred embodiment of the present invention, the aqueous medium is an aqueous stream, and the source of conductivity increasing substance is calcium hypochlorite, which is introduced into the aqueous stream through the use of a feeder unit, i.e., a chlorination unit.