The U.S. Bureau of Mines studied the hydrolysis of fe(III) in no3- and so2/4- solutions at different temperatures to determine the conditions under which dense fe(III) precipitates form. Solutions of known composition made from reagent chemicals were maintained in controlled-temperature baths, and changes in color and formation of solid phases were observed as a function of time. Solids were isolated and characterized by x-ray powder diffraction or infrared spectroscopy. Dense fe(III) precipitates are obtained in fe(no3)3 solutions especially at temperatures greater than or equal to 60 deg c in acidified solutions. At lower acidities, colloids, a cationic hydrolytic polymer, or gelatinous precipitates form. In so2/4- - containing solutions a dense amorphous basic ferric sulfate (dabfs) of compositions fe4(oh)10 so4.Xh2o (x = 2 to 3) forms at lesser than or equal to 40 deg c from solutions with so2/4- and fe3+ concentrations lesser than or equal to 0.2 M and mole ratio so2/4- /fe(III) greater than or equal to 0.5. Chemical observations showed that dabfs is related to the fe(III) cationic hydrolytic polymer.