The effect of ferrous and ferric iron on the crystallization of calcium sulfate dihydrate, the rate of filtration, and the precipitation of sludge in concentrated acid were investigated. A bench scale unit was used to simulate dihydrate plant conditions. It was found that ferrous iron was oxidized to ferric, and the behavior of the two impurities in the acid was therefore very similar. At low levels of impurities, ferrous and ferric iron improved filtration, but further additions decreased it. Sludge collected from the concentrated acid after five weeks was mainly calcium sulfate hemihydrate and anhydrate. With the addition on 0.5% and 0.75% iron impurities however, the presence of (Fe,Al)(3)KH14(PO4)(8).4H(2)O was evident in electron micrographs.