A study of the fractional precipitation of the rare earth iodates from homogeneous solution using double complexation and replacement has been made. The Doerner-Hoskins logarithmic distribution coefficient (lambda) has been found for a variety of combinations of two rare earths and two complexing agents. Lambda values obtained using two complexing agents have been compared with lambda values obtained using only one complexing agent. Additional lambda values have been calculated from these experimentally determined values. A comparison of calculated and experimental values are in good agreement. Approximate lambda values have also been computer calculated with a set of equations using solubility product and formation constant data.COPRECIPITATION effects have complicated fractional precipitation of the rare earths. Precipitation from homogeneous solution in the presence of a complexing agent has reduced coprecipitation ( I ) . This study shows that when two complexing agents are present further improvement has often been realized.To organize and interpret information on a fractional precipitation process, results are expressed using the Doerner-Hoskins logarithmic distribution coefficient or lambda. If the system under study obeys the Doerner-Hoskins equation, then the lambda will be the same for a different starting ratio of two ions to be separated or if the amount precipitated varies. This provides a handy means of evaluating what would otherwise be confusing and varied results.The application of the Doerner-Hoskins equation (2) to coprecipitation studies usually involved oxalates (3). Only limited work on rare earth iodates has been previously published (4-6). This study is similar to that previously reported for lanthanum and praseodymium (4) and represents the application of double complexation and replacement to many other rare earths. It also shows that double complexation often provides a superior separation over single complexation. EXPERIMENTAL Reagents.Analytical Grade reagents were used, except as described below.Rare earth oxides were purchased from two sources: Baker and Adamson, Allied Chemical, Morristown, N. J., and Alfa Inorganics Inc., Beverly, Mass. These oxides were generally of 99.9% purity with an occasional oxide of 99.8 %.Three of the complexing agents used in this study were (1) L. Gordon, M. L. Salutsky, and H. H. Willard, "Precipitation (2) H. A. Doerner and W. M. Hoskins, J . Am. Chem. SOC., 47,