The uncompleted solubility curve for natural ferrimolybdite by Vinogradov (1957) was completed with good correlation for both natural ferrimolybdite and a synthetic product with identical x-ray diffraction pattern. An area of lowest solubility for natural ferrimolybdite was established at approximately 3.35 pH. This value was slightly more basic for synthetic material. The synthetic material was also less soluble throughout the whole pH range. An area immediately to the more basic side of the least solubility point was complicated by a possible iron-molybdenum complexing. In the low pH range, both natural and synthetic material showed apparent complete solubility below 1.3 final pH.