A modification of the EDTA method of determining ion activities in soil developed by Norvell was used to measure the solubility of various ferric hydroxides. The solubility products of jarosite (KFe3(SO4)2(OH)6) and amorphous Fe(OH)3 were also determined on a natural jarosite nodule taken from an acid sulphate soil.For amorphous ferric hydroxide and lepidocrocite, the method gave pK° values of 39.02 ± .35 and 42.48 ± .23, respectively. Hematite and goethite were too inert to yield reproducible solubilities.For a mixture of jarosite and ferric hydroxide, the respective pK° values were 98.56 and 39.93. These values were used to develop an Eh‐pH stability diagram that helps to explain the occurrence of jarosite under acid oxidizing conditions in soils.