Monopersulphates, e.g. of sodium, potassium, calcium, magnesium, zinc, aluminium and ammonium and produced by neutralizing an aqueous solution of monopersulphuric acid to a pH not exceeding 3 (see Group III). Alkali metal persulphates thus produced may be rendered free-flowing by the addition of one or more boron compounds, e.g. borax, boric acid, boric oxide or perborates to the extent of 1-12 per cent. In addition, extenders and stabilizers mentioned and/or exemplified are sesquicarbonates, sodium carbonate, phosphates and polyphosphates, sulphates and silicates. Stable compositions may also be obtained by drying a solution of potassium mono-persulphate containing potassium hydrogen sulphate and potassium sulphate.ALSO:Monopersulphates are produced by neutralizing an aqueous solution of monopersulphuric acid to a pH not greater than 3. The neutralization may be carried out with a metal hydroxide or carbonate, e.g. the hydroxides or carbonates of sodium, potassium, magnesium and calcium or ammonium hydroxide or carbonate or the hydroxides of zinc and aluminium, which may be used in the form of an aqueous solution or slurry or as a solid: if the metal forms a relatively insoluble sulphate it may be removed by filtration. The neutralization should be carried out at or below 40 DEG C., preferably between -10 DEG and 10 DEG C. The monopersulphuric acid is preferably produced by hydrolysing persulphuric acid containing 20-40 per cent by weight of electrolytically produced disulphuric acid at 40-80 DEG C., the hydrolysis being terminated before the concentration of hydrogen peroxide reaches 1 per cent by weight. The preferred initial concentration of sulphuric acid for the electrolysis is a 35-50 per cent solution. The time of hydrolysis varies with the temperature used between about 3 and 30 minutes and may be terminated by reduction of the temperature of the mixture and/or by raising the pH of the solution by effecting the neutralization to produce monopersulphate. It is stated that completely to prevent hydrolysis solely by cooling, temperatures of about -40 DEG C. are necessary. After neutralization, the solution may be concentrated by evaporation under vacuum, filtered and dried. In an example of the production of permonosulphuric acid, 45.3 per cent sulphuric acid was electrolysed to give a product containing 27.6 per cent of persulphuric acid. The product was cooled to 10-20 DEG C., transferred to a jacketed hydrolysis vessel and there heated to 30-35 DEG C. by means of hot brine. After two hours the mixture was chilled to 0 DEG C. to give a solution containing 16.6 per cent of H2SO5. Other suitable hydrolysis conditions are indicated in a table. The alkali metal persulphates may be obtained in free flowing condition by the addition of a boron compound, preferably boric oxide, boric acid, and alkali metal borates, e.g. borax, or perborates, e.g. sodium perborate, to the extent of 1 to 12 per cent. Extenders which may be used in addition include sesquicarbonates, phosphates, sulphates and silicates. The boron compound is preferably supplied whilst the monopersulphate is in solution and may be mixed with the neutralizing agent and added during the neutralization, or added separately during or after neutralization. After the addition of the extender drying may be performed either by vacuum drying or preferably by spray drying, e.g. with an inlet temperature of 343 DEG C. and an outlet temperature of 71 DEG C. Stable compositions may also be obtained by drying a solution of potassium monopersulphate containing potassium hydrogen sulphate and potassium sulphate, said composition having been obtained by neutralization of a monopersulphuric acid in the presence of sulphuric acid with potassium carbonate or potassium hydroxide, the potassium carbonate or hydroxide being employed in an amount of 75 to 90 per cent of that required to raise the pH to 3. Alternatively the potassium sulphate and bisulphate may be added to a solution of persulphate produced in accordance with the invention. When neutralization is employed for the production of the compositions, it is preferred to use a monopersulphuric acid which has been produced by reacting oleum with hydrogen peroxide, the oleum preferably consisting of sulphuric acid containing 65 per cent of sulphur trioxide and the hydrogen peroxide preferably containing 35 to 90 per cent of H2O2. Molar ratios of H2SO4 : H2O2 of 1 : 1 to 1 : 8 are preferably used. The Specification also refers to the tray-drying in a forced-draught oven of a KHSO5-KHSO4-K2SO4 slurry and to the use of a vacuum desiccator after washing with cold absolute ethanol. Pure potassium monopersulphate obtained in an example was in the form of water-white efflorescent platelets melting at 100 DEG C. with decomposition. In the examples relating to aluminium and zinc monopersulphates, a slurry of the appropriate freshly precipitated hydroxide was added to a 10 per cent solution of monopersulphuric acid and solid barium carbonate was thereafter added to bring the pH of the solution up to 2.5. The barium sulphate was filtered off and the filtrate evaporated to dryness. In the examples relating to ammonium, calcium and magnesium monopersulphates, the reagents used were in each case the solid carbonates. In other examples, sodium monopersulphate after production was stabilized with a mixture of tetrasodium pyrophosphate, anhydrous borax, sodium carbonate and sodium tripolyphosphate, or a mixture of sodium sesquicarbonate and anhydrous borax, a mixture of borax and boric oxide, a mixture of sodium tripolyphosphate, tetrasodium pyrophosphate and boric oxide or with borax alone, and potassium monopersulphate was stabilized with boric acid or sodium borate.