The zincate immersion process is a commercial process used to treat aluminium prior to electroplating. Superior zinc coatings are obtained through modifications to the original process. One such modification involves the addition of ferric chloride, complexed with potassium sodium tartrate, to the plating baths. This paper describes a detailed investigation into the function of these additives. It was found that the addition of tartrate a:lone did not significantly affect the rate of zinc deposition, the deposit morphology or the position of the aluminium or zinc polarization curves. This suggests that the role of tartrate is solely as a complexing agent to maintain iron (III) in solution. In contrast to this, the addition of iron(III)/tartrate had a marked effect on the reaction. It was found that iron(III) functioned primarily by reducing the size of the zinc crystals as they formed. This produces a thinner and more compact zinc immersion coating.