The present study documents the precipitation of Fe(III), silica, and sulfate in the presence of 3 different bacteria(Bacillus subtilus, Bacillus licheniformis, andPseudomonas aeruginosa), under different total Fe(III) concentrations (10−2M, 10−3M, 10−4M) at constant pH (4.0). Morphology and chemical composition of the precipitates were compared with those formed in abiotic control systems, while chemical composition and precipitation of the precipitates were modeled according to solution chemistry data. Transmission electron microscopy (TEM) observations showed morphological differences between the biotic and abiotic systems. All systems contained small grains (diam. 2–50 nm), but amorphous material (i.e., material without any specific morphology) and nodules were present only in the cell systems. This is because bacterial surfaces and exopolymers provided numerous binding sites for metal and anion sorption and promoted heterogeneous nucleation of hydrous ferric oxides (HFO). The initial Fe/Si and Fe/SO4molar ratios of the solutions dictated the type of precipitates in most systems, since abiotic control systems were saturated to oversaturated with respect to amorphous silica, siliceous ferrihydrite, schwertmannite, ferrihydrite, goethite, or combinations of these. Of the three strains studied,B. licheniformisappeared to have the greatest influence on the chemical composition of the precipitates, especially in the presence of Si.B. licheniformis(a gram‐positive bacterium with a large capsule) favored the precipitation of HFO containing less Si than the predicted solids, because Si rather than Fe oxides was preferentially sorted to extracellular polymers (capsule). On the other hand, the formation of SO4‐rich HFO (similar to schwertmannite) did not seem to be affected by the presence of bacteria.