The dithiocarbamates are decomposed on heating with hydrochloric acid in the presence of stannous chloride. The carbon disulfide (CS2) which is formed, is transferred with a current of nitrogen, into a trap containing concentrated sulphuric acid to remove interfering substances, and then into a trap containing a methanolic solution of potassium hydroxide. The concentration of potassium o-methyl dithiocarbonate formed under these conditions is measured by spectrophotometry. As it is not normally known which dithiocarbamate is present in tobacco, results are expressed as carbon disulphide, and the values obtained using this method are taken to indicate the dithiocarbamate residue level.