This paper compares the scale inhibition efficiencies of a phosphate ester, two phosphonate and two polymer chemistries in the presence of ferrous iron, calcium carbonate, barium sulfate, iron sulfide and iron carbonate scales, as well as combinations therein. The results show that as the scale speciation of the precipitating solids change so does the scale inhibition efficiency. These changes in efficiency were attributed to the cumulative effects of crystal matrix distortion by either exogenuous scale specie or effect of metal complexation to the scale inhibitor chemistry. The ultimate conclusion of this study is that because of the relationship between the scale inhibitor performance and ferrous iron concentration, it is important to consider this relationship when determining the minimum inhibitor concentration (M.I.C.) for the inhibition of scale growth in oilfield production systems.