The purpose of this study was to model, based on thermodynamic equilibrium constants, the effects of chloride and phosphate ion on the speciation of ferric iron in solution and on Fe(III)-precipitates. The thermodynamic modelling was based on the geochemical modelling code PHREEQC and the thermodynamic database WATEQ4F. Increasing phosphate levels (g per L range) increase the complexation of ferric ion with phosphate (FeH2PO42+) with a parallel decrease in ferric sulphate complex (FeSO4+) and release of sulphate as SO42- in solution. Chloride ion at comparable levels and under otherwise similar conditions had negligible effects on the speciation of soluble iron species. In the solid phase analysis, jarosite and goethite species declined with increasing phosphate levels, whereas chloride did not affect the relative proportions of secondary Fe(III) minerals in the solid phase. Saturation index values for jarosites and goethite were dependent on the temperature with the range of phosphate levels (0–20 g/L) examine in this study.