The invention concerns novel phenoxy- and phenylthio-acetyl derivatives of (4-amino-2,6-dimethylphenylsulphonyl)nitromethane and pharmaceutically acceptable salts thereof which are inhibitors of the enzyme aldose reductase and are of value, for example, in the treatment of certain peripheral effects of diabetes and galactosemia. Also disclosed are pharmaceutical compositions containing one of the derivatives and processes for the manufacture and use of the derivatives.