Iron can be precipitated out of leaching solutions by using high pressure processes. The focus of this study is on the various iron phases forming during the pressure oxidation of sulfates. Artificial solutions were produced from sulfuric acid, copper and iron sulfates in an attempt to recreate the matrix composition and conditions used for copper sulfides autoclaving. The influence of the following factors was investigated: initial free acidity, initial copper concentration and initial iron concentration. There were three solid species formed in the autoclave: hematite, Basic Iron Sulfate (BIS) and hydronium jarosite. Free acid is the main factor influencing the composition of the residue. The increase of BIS content in the residue is not gradual and occurs over a change of a few grams per liter of sulfuric acid. Increasing copper sulfate concentration in the solution hinders the formation of BIS. This effect seems to be related to the buffering action of copper sulfate, decreasing the overall acid concentration and thus extending the stability range of hematite. Increasing iron concentration on the precipitate chemistry seems to promote BIS formation. At high iron level, the most noticeable effect is the inhibition of jarosite. The results were reported within a Cu-Fe-S ternary system and modeled. The modeling confirmed the experimental observations. (C) 2016 Elsevier Ltd. All rights reserved.