A simple and economical process for the preparation of polyphosphates of organic bases consists of reacting a mixture of phosphorus pentoxide and at least one organic nitrogen base with at least one compound which releases water accompanied by decomposition under the prevailing conditions in such a molar ratio that upon decomposition of the water-releasing compound at most essentially 2 mol. water are produced per mol. phosphorus pentoxide. Thus-prepared polyphosphates are particularly suitable as flame-protection agents for plastics.