Hydrous aluminum oxide particles precipitated from simulated wastewater were examined using atomic force microscopy in order to understand their structural and coating properties. Three types of particles were examined : aluminum oxides coprecipitated by adding alum in the presence of a solution of phosphates and tannic acid; postprecipitated particles formed by adding phosphates and tannic acid to already formed aluminum oxides; and a control case consisting of particles precipitated in the absence of either phosphate or organic component. Using tapping mode and phase imaging atomic force microscopy, it was found that the postprecipitated particles had distinctly different viscoelastic properties than either of the other two particle types and also varied markedly in particle size and morphology. These observations are consistent with a model in which the postprecipitated particles are coated with an organic coating of tannic acid. The results are discussed in the context of the relative effectiveness of these aluminum oxides in removing phosphates and other contaminants from wastewater during sewage treatment.