The present invention relates to water treatment. In one embodiment 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 comprises adding treatment agents to said aqueous system and wherein said treatment agents comprise: (a) a phosphonium compound and (b) an quaternary ammonium salt (b) having formula:whereinR2 represents a C1-C6 group which is saturated or unsaturated and which is unsubstituted or substituted by a halogen andX represents either chlorine or bromine.
