The invention relates to process for making phosphorus pentoxide by subjecting elementary phosphorus to a combustion reaction with the aid of air, the reaction heat being utilized for the production of energy. To this end, the invention provides for the phosphorus to be subjected to combustion with dry air containing, per m3, 5 g down to 0.01 g of water, inside a combustion chamber of which the metallic walls are arranged so as to form a cooling system with cavities therein and for a liquid and liquid/steam-mixture, respectively, to be circulated through the cooling system under pressure within the range 1 to 150 bars, the liquid or mixture assuming a temperature of more than 150 DEG C. up to 500 DEG C. Steam which is formed in the cooling system is continuously taken therefrom and replaced by an equivalent proportion of fresh liquid.