Thin-layer of magnesium hydrogenphosphate was prepared over a glass plate by a sol-gel technique using sodium diphosphate and magnesium nitrate to be employed for the continuous removal and recovery of aqueous ammonium. Although it had been reported before that magnesium hydrogenphosphate was prepared with sodium alginate together with sodium diphosphate and magnesium nitrate in the sol-gel technique, the present study revealed that the employment of sodium alginate resulted in the formation of a thin-layer of magnesium hydrogenphosphate that could be easily peeled off from the glass plate. The preparation procedure described in the present study afforded a rather stable thin-layer of magnesium hydrogenphosphate which does not peel off from the glass plate even after repeated use in the removal and recovery of aqueous ammonium. The possibility for the employment of the thin-layer of magnesium hydrogenphosphate for repeated use of the removal and recovery of aqueous ammonium was described.