Abstract: The present invention relates to a process for the oxidation of alkali metal phosphites to alkali metal phosphates at temperatures of between 400.degree. and 500.degree. C. To this end, alkali metal phosphites are employed in the form of an aqueous solution or suspension and, if necessary, the alkali metal : P atomic ratio in this feed material is adjusted to a ratio which at least corresponds to the atomic ratio desired for the final product.More particularly, the solution or suspension is spray-dried in a tower, in a single operational stage, by spraying in a flame zone. Starting material free from alkali metal phosphate and with an alkali metal : P atomic ratio equal to or greater than 3.5 is used for making a trialkali metal phosphate which is obtained in admixture with an alkali metal hydroxide. Starting material which contains from 25 to 45% by weight of alkali metal phosphate, calculated as phosphorus, based on their total phosphorus content, is employed for making alkali metal polyphosphates.