A range of effective optimal formulations of polyferric sulphate (PFS) can be prepared by a novel procedure, recently developed by the authors, involving an oxidation stage followed by a partial hydrolysis (aging) stage. The Fe(III) concentrations, the dose and nature of the base used, and the temperature and time of partial hydrolysis influence the properties of the resulting PFS formulations and these have been carefully studied. The analytical techniques of membrane ultrafiltration, ferron-timed spectroscopy and specific electrophoretic mobility have been used to characterise the nature of the PFS species. The optimal PFS coagulating species have been found to have medium molecular weight (0.5-10 kDa) and high cationic charge. Such optimal species are believed to be responsible for the superior performance of PFS in water treatment practice.