Abstract: Aqueous phorphoric acid solutions are produced by contacting an elemental phosphorus-containing material with an oxygen-containing gas, while both are finely divided and well mixed together in a reactor vessel, while the phosphorus-containing material is otherwise protected from direct contact with oxygen in the atmosphere by being blanketed with a protective aqueous layer, and removing and recovering the heat generated in the reactor vessel (e.g., by means of a heat exchanger) due to the contact between the oxygen in the oxygen-containing gas and the elemental phosphorus in the elemental phosphorus-containing material. The technique is useful for the production of phosphoric acid from substantially pure elemental phosphrous.