The invention relates to a simple, cost-effective method for producing polyphosphates of organic bases. According to said method, a mixture of phosphorus pentaoxide and at least one organic nitrogen base is reacted with at least one compound, which in the prevailing conditions releases water as it decomposes, in such a molar ratio that during the decomposition of the water-releasing compound a maximum of essentially 2 mols water per mol of phosphorus pentaoxide is produced. Polyphosphates produced in such a way are particularly suitable for use as flame-retardant agents for plastics.