Urea or biuret was added to the thermal synthetic system of Rhabdophane-type neodymium and cerium phosphates. The mixture of a rare earth compound, a phosphorus compound, and an additive [CO(NH₂)₂ or NH(CONH₂)₂] was heated at 150°C or 300°C for 20 hr, and the thermal products were analyzed by the XRD, FT-IR, and BET methods. H₃PO₄ and (NH₄)₂HPO₄ were used for phosphorus compounds, and for rare earth compounds, Nd₂O₃, Nd(NO₃)₃ · 6H₂O, NdCl₃ · 6H₂O, Nd₂(CO₃)₃ · 8H₂O, CeO₂, Ce(NO₃)₃ · 6H₂O, CeCl₃ · 7H₂O, and Ce₂(CO₃)₃ · 8H₂O were used. Urea and biuret worked not only as a dispersing agent but also as a reactant. By the addition of biuret, the thermal products changed from cerium oxide to Rhabdophane-type cerium phosphate in the system using CeCl₃ · 7H₂O and (NH₄)₂HPO₄. Addition of urea or biuret influenced the specific surface area of Rhabdophane-type neodymium and cerium phosphates. Furthermore, to increase the reactivity of the raw solid materials, mechanical treatment was performed. The mixture of diammonium hydrogenphosphate and a rare earth compound was ground with water and then heated. The influence of the addition of urea or biuret was also studied in these systems.