a process for the production of phosphoric acid esters or phosphoric acid ester mixtures of polyhydric alcohols by reaction of the polyhydric alcohols with phosphorus pentoxide is characterized in that the amount of phosphorus pentoxide which is required for the esterification operation is suspended in an amount of the ester or ester mixture to be produced , thereupon the amount of polyhydric alcohol required for the esterification operation and possibly water is added at a temperature in the range of between 20 and 180 ° c ., thereupon the esterification reaction is substantially completely executed and thereafter the product is obtained . the products of the process are used for the production of flame - proofing treatments .