A method of determining the TOC of a purified water stream is disclosed comprising at least the steps of: passing a supply water stream through a water purification apparatus including an oxidizer to provide a purified water stream, and dispensing at least a portion of the purified water stream; stopping the dispense of the purified water stream and recirculating the purified water stream as a recirculating water stream through at least a portion of the water purification apparatus including the oxidizer to provide a re-oxidized water stream; and measuring the conductivity value of the re-oxidized water stream to determine the TOC of the purified water stream. In this way, a determination of the TOC of the purified water stream of can be provided without requiring a dedicated TOC monitor at the point of dispense of the purified water stream.