This paper describes a new water-insoluble molybdenum compound that has been developed as a slow-release fertilizer. The compound is an inorganic polymer formed by inclusion of molybdenum within a long-chain polyphosphate structure. It was designed by a process of "reverse engineering" of the molecule. Synthesis involved reaction of phosphoric acid with magnesium oxide, molybdenum trioxide, and sodium carbonate at 275 degrees C. Kinetics of reaction revealed complex multistage processes. X-ray diffraction patterns showed a crystalline nature with short-range as well as long-range ordering. The magnesium sodium polymolybdophosphate had ideal slow-release characteristics; it had low water solubility and high citrate solubility and was powdery, free flowing, and nonhygroscopic. Field testing showed an 80% increase in yield of green gram at a low dose of 0.04 kg/ha Mo. Nodulation increased by over 161%, and N content of gram increased by 20%. The slow-release fertilizer would provide an effective, low-cost, and environmentaly friendly alternative to Mo fertilization.