Abstract: A process is provided for preparing calcium sulphate plaster compositions having water-retaining and retarding characteristics, and comprising a water-soluble cellulose ether prepared by reacting cellulose mercerized with alkali hydroxide, and suitable reactants and a retarder. The mercerized cellulose still contains alkali impurities and an acid, the alkali salt of which is adapted to serve as calcium sulphate plaster retarder is added in an amount substantially equivalent to the amount of alkali hydroxide remaining in the cellulose ether.Calcium sulphate plaster compositions also are provided as obtained by this process.