Demand for improved battery systems for electric vehicles and power sources in general suggests the need for more accurate descriptions of the solid-state and electrochemical reactions which may affect active-material usage. Tetrabasic lead sulfate (4PbO . PbSO4) is an intermediate phase commonly formed during production of lead-acid batteries and, with tribasic lead sulfate, determines some battery characteristics. Single-crystal X-ray data have been used to determine its structure to a final weighted R of 0.024 (a = 7.297, b = 11.698, and c = 11.498 Angstrom, beta = 90.93 degrees, P2(1)/c). Its structure is closely related to that of tetragonal PbO, except that one of six Pb sites is occupied by SO4. This sulfate group shows either dynamic (librations) or static disorder, but there is no evidence in our crystal for Pb-S disorder as proposed by other X-ray studies. This determination and that for tribasic lead sulfate complete structural determinations of the major phases formed during production of lead-acid batteries. These structural models allow Rietveld studies using either X-ray or neutron-diffraction data to evaluate properties such as crystallinity, atomic ordering, and reactions during curing and cycling. The structures of tetrabasic and tribasic lead sulfate, as well as tetragonal and orthorhombic PbO, have basic features in common. These similarities suggest that there may be structural control during some reactions whereby heterogeneous nucleation influences the products in concert with external variables such as temperature, humidity, and pH.