The invention comprises 5-sulphanilamido-2:4-dimethyl pyrimidine and its preparation by condensing acetamidene or a salt thereof with an a -alkoxymethylidene-acetoacetic ester such as ethyl a -ethoxymethylidene-acetoacetate, preferably in aqueous solution in the presence of an alkali, for example potassium carbonate, at about 20 DEG C., or in alcoholic solution in the presence of an alkali-metal alkoxide, treating the resulting 2:4 - dimethyl - pyrimidine - (5)-carboxylic acid ester by known methods for the conversion of a carboxylic acid ester into an amine, to convert the ester group into an amino group, reacting the resulting 5-amino-2:4-dimethyl-pyrimidine with benzene-sulphonyl halide which bears in the p-position a group convertible to an amino group by hydrolysis or reduction, such as an acetamino group or a nitro group and converting the p-substituent in the resulting sulphonamide, for example in 2:4 - dimethyl - 5 - (N4 - acetyl - sulphanilamido) pyrimidine or 2:5-dimethyl-5-(4-nitrophenylsulphonamido)-pyrimidine to give the desired sulphanilamide. The conversion of the 2:4 - dimethyl - pyrimidine - (5) - carboxylic acid ester into the corresponding amino compound is suitably effected by treating with ammonia to give the 2:4-dimethyl-pyrimidine-(5)-carboxylic amide, reacting with bromine in the presence of sodium methoxide at an elevated temperature to give an N-[2:4-dimethyl-pyrimidine - (5)] - carbamic acid ester and hydrolysing this by treatment with alkali. Examples are given. Starting materials. Alkyl esters of a -alkoxymethylidene-acetoacetic acid such as ethyl a -ethoxymethylideneacetoacetate are made by reacting the appropriate alkyl acetoacetate with an alkyl orthoformate. Specification 800,777 is referred to.