The current invention relates to a process for stabilising a phosphorus-containing compound aqueous composition, comprising the step of adding to the composition an efficient arsenic stabilizing amount of a compound selected from the group consisting of ammonia, ammonium salt, organic amino acid, peptide and polypeptide application of the stabilized composition for treating an aqueous system optionally containing or in contact with metal sulphide scale, which method comprises adding to the system, separately or together, an efficient anti-scale amount of a stabilized aqueous composition or for treating a water system to kill or inhibit the growth of micro organisms comprising applying thereto or forming in situ an efficient inhibiting amount of the stabilized aqueous composition.
