A method may include treating a water system containing or in contact with a metal sulphide scale to inhibit, prevent, reduce, dissolve or disperse iron sulphide deposits. A solution of tris(hydroxyorgano)phosphines (THP) and/or tetrakis(hydroxyorgano)phosphonium salts (THP+salts) and citrate is contacted with the metal sulphide scale thereby to dissolve at least part of said scale in said solution.
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