A starting procedure for the production of phosphoric acid in a multi vessel system comprises filling at least two communicating vessels (16, 28) with phosphoric acid, adding phosphate rock to a third vessel (2) that communicates with one (16) of the two vessels and adding sulphuric acid to the second (28) of the two vessels. Fluid is passed from the two vessels to a fourth vessel (48) as the volume of fluid increases by reaction and the generation of foam, and fluid from the fourth vessel is passed to the third vessel. Fluid is also circulated from the first to the second of the two vessels. Part of the contents of the second of the two vessels is removed and separated into phosphoric acid and calcium sulphate, part of the phosphoric acid being returned to the third vessel.