Process for producing methyl hydroxyalkyl cellulose ethers

In a process for producing methyl hydroxyalkyl cellulose ethers by alkalisation and etherification of celluloses and rinsing the methyl hydroxyalkyl cellulose ethers with water and recovering the short-chain cellulose ethers that are soluble in the wash water, the improvement which consists of passing the wash water containing sodium chloride and organic by-products at temperatures from 30.degree. to 90.degree. C. across a first stage of a membrane filtration apparatus having a molecular cut size of 200 to 150,000 Dalton, thereby concentrating the wash water, mixing the concentrate thereby obtained with 1 to 30 times its quantity of water diluting this concentrate with water add passing it at temperatures from 30.degree. to 95.degree. C. across a second stage of a membrane filtration apparatus having a molecular cut size of 200 to 150,000 Dalton, again concentrating and adding the concentrate which is substantially free from sodium chloride and organic pollutants to the washed methyl hydroxyalkyl cellulose ethers, or alternatively recovering as solid substances the methyl hydroxyalkyl cellulose ethers of low viscosity that are contained in the concentrate.