The reaction between PbO and H2SO4 in water leading to the formation of tribasic lead sulfate hydrate was studied using pH measurements, x-ray diffraction, scanning and transmission electron microscopy, and infrared spectroscopy. We examined the effect of different parameters such as temperature, starting composition, Pbo source, order of the materials addition, and presence of carbonates, on PbO-H2SO4 reaction. From this study, a mechanism is proposed for the reaction. It explains why, by mixing PbO and H2SO4 in water, the tribasic lead sulfate cannot be obtained as a pure phase. Some unreacted PbO always remains at the end of the reaction.