Embodiments of the present subject matter provide a biocidal composition comprising an aqueous mixture of (a) a phosphonium compound and (b) a polymeric ammonium compound, wherein the weight ratio of compound (a) to compound (b) is from 0.2:1 to 20:1, and whereinthe phosphonium compound (a) has formula:wherein each R is independently a C1-C6 alkyl group which is unsubstituted or substituted by a cyano, hydroxyl, esterified hydroxyl or aryl group；R1 represents a C8-C18 alkyl group which is substituted or unsubstituted； andX represents either chlorine or bromine； and whereinthe polymeric ammonium compound (b) comprises the repeat unit:wherein each R2 is independently a C1-C2 alkyl group which is substituted or unsubstituted；R3 represents a C2-C18 alkyl or alkenyl group which is substituted or unsubstituted； andR4 represents a C2-C18 alkyl or alkenyl group which is substituted or unsubstituted, a diethyl ether group, an isopropanol group, a N,N-dipropylurea group, or a 2-butene group.In some embodiments, the biocidal composition has a synergy index (SI) of less than 1 in relation to the effect of the composition on the inhibition of growth of biological organisms.
