The present invention relates to water treatment. In one example, there is provided a method of treating an aqueous system to inhibit growth of one or more micro-organisms therein and/or to reduce the number of live micro-organisms therein. The method includes adding treatment agents to an aqueous system wherein said treatment agents include:(a) a phosphonium compound； and(b) a compound having formula:M(XO2)n wherein:M is a Group I or Group II metal；X is a halogen； andn is 1 or 2.
