The dienes CH2=C(CH2Cl),CMe=CH2 (3), CHCl=CMe.CMe=CH2 (4), CHCl = CMe. CMe = CHCl (14), and CHCl = C(CH2Cl). CMe = CH2 (21) have been synthesised via chlorination of 2,3-dimethylbuta- 1,3-diene. Copper-catalysed addition of benzenesulphonyl chloride to 2,3-dimethylbuta-1,3-diene and diene (3) gives the 1,4-adducts PhSO2. CH2. CMe = CMe. CH2Cl and a mixture of PhSO2.CH2.C(CH2Cl)-CMe.CH2Cl and PhSO2. CH2. CMe = C (CH2Cl)2, respectively. Dehydrochlorination of the former adduct gives the diene PhSO2CH = CMe.CMe = CH2 which polymerises in air. Dienes (4) and (21) form Diels-Alder adducts with tetracyanoethylene. Chlorination of diene (14), is surprisingly accompanied by addition of hydrogen chloride and gives the compounds CHCl2.CClMe.CClMe.CH2Cl and CHCl2.CCl(CH2Cl). CClMe.CH2Cl. The dichloride CH2Cl. CMe = CMe. CH2Cl reacts with an equimolar amount of bromine by addition, but with an excess of bromine the products are CH2Br.C(CH2Cl) = C(CH2Cl).CH2Br(89%) and (CH2Br)2C = C(CH2Cl).CH2Br (6%). Unexpected substitutive bromination is observed on treatment of the trichloride (CH2Cl)2C=CMe.CH2Cl with bromine to give (CH2Cl)2 C = C(CH2Cl).CHzBr (95%).