Phosphate derivatives of a 1,3,5-triazine compound, preferably melamine polyphosphate, which are heat-resistant at the processing temperature, are selected which have an average condensation coefficient n (number average)>20, a pH-value of a 10% slurry of the polyphosphate derivative in water at 25° C. of 5 or higher, a molar ratio of 1,3,5-triazine compound to phosphorus (M/P) 320° C. They can be produced by heat treatment of an orthophosphate or condensed phosphate with an average condensation coefficient n (number average) below 20 in an ammonia atmosphere at a temperature in the range of 300 to 400° C. until the average condensation coefficient is above 20 and the molar ratio of 1,3,5-triazine compound to phosphorus (M/P) is below 1.1. The polyphosphates of the invention can be represented in simplified form by the following general formula: wherein M denotes a 1,3,5-triazine compound and n denotes the average condensation coefficient. For high and medium condensation coefficients n the sum formula can be reduced to (MHPO3)n. Therein M once again denotes the 1,3,5-triazine compound and n denotes the average condensation coefficient. These polyphosphate derivatives can be used as flame-retardant agents in any plastic materials, preferably thermoplastic materials and thermosetting materials, in particular also in glass fibre-reinforced polyamides and polyesters which are processed at high temperatures.