Hexahalocyclopentadienes are reacted according to the Diels-Alder method with diolefinic compounds comprising uninuclear or condensed multinuclear partly hydrogenated aromatic ring systems in which each nucleus contains a 1 : 4-methylene bridge; the products have the skeleton <FORM:0692546/IV (b)/1> in which one of the terminal 5-membered rings has a double bond and only halogen atoms are attached to the carbon atoms in this ring. The other terminal ring also contains a double bond, and this may be further reacted with chlorine or bromine to give addition products which may then be dehydrohalogenated. The Diels-Alder reaction is carried out by heating the reactants together, with or without a solvent. Either one or two molecules of halocyclopentadiene react with the diolefine, depending on the proportions of reactants. In the examples: (1) hexachlorocyclopentadiene is reacted with bicyclo - (2 : 2 : 1) - 2 : 5 - heptadiene to give hexachlorotetracyclodecadiene and dodecachlorohexacycloheptadecadiene (in one or two stages); (2) hexachlorocyclopentadiene is reacted with 2 : 3-dicarbethoxybicyclo-(2 : 2 : 1)-2 : 5-heptadiene (obtained from dicarbethoxyacetylene and cyclopentadiene) to give dicarbethoxy - hexachlorotetracyclodecadiene; (3) hexachlorotetracyclododecadiene is additively brominated or chlorinated to give a mixture of two stereoisomeric octahalo-compounds; (4) the products of (3) are dehydrohalogenated to heptahalo-compounds. Hexabromocyclopentadiene is also mentioned as reactant. Specification 614,931 is referred to. The Specification as open to inspection under Sect. 91 includes the subject-matter of Specifications 692,545 and 692,547 and mentions compounds similar to those of the latter Specification where the epatomic component is >SO, >NH, >NR or the ionized groupings >SH, >SR, >NH2 and >NHR; R is an alkyl or aryl group, or any other group that may be attached to a nitrogen or sulphur atom. In all the types of compounds described the following may be present as substituents in any ring except that containing the six halogen atoms:-R, Cl, Br, I, OH, OR, SH, SR, NH2, NHR, NR2, NR3 (ion), COR, OCOR, COOH, COOR or CN, R being a hydrocarbon or substituted hydrocarbon group. These substituents may be present in the starting materials or may be introduced into the products. In addition, the substituents on the methano bridges may be alkylidene or cycloalkylidene, e.g. when a fulvene is used as dienophile. Compounds may be obtained from the above by loss of H2O, ROH, H2S, RSH, NH3, NH2R, NHR2 or H Hal or by tautomeric shift. Probable stereo-chemical formul are given for the products. Further reactions mentioned are: (1) 6 : 7-epoxy - hexachlorotetracyclododecene is heated with acetic acid and a catalyst (e.g. sulphuric acid) to give a hydroxyacetate; this is converted by phosphorus tribromide (or other reagent) into a bromo-acetate; the latter when treated with alkali is hydrolysed and ring-closed to a stereoisomer of the starting material; (2) 6 - keto - hexachlorotetracyclododecene reacted with p-chlorophenylmagnesium bromide gives p - chlorophenyl - hydroxy - hexachlorododecene (sic) which loses water on heating to yield p - chlorophenyl - hexachlorotetracyclododecadiene. This subject - matter does not appear in the Specification as accepted.ALSO:Insecticidal compositions contain as active ingredient one or more compounds of the structure <FORM:0692546/VI/1> in which one of the terminal 5-membered rings has only halogen atoms (chlorine or bromine) attached to the carbon atoms and contains a double bond. The compounds may be dissolved in oils and/or emulsified in water, or combined with carriers to give wettable or non-wettable dusts. They may be added to whitewashes, paints, lacquers, varnishes or waxes to give insecticidal surfaces, or used to impregnate paper or plastic materials to protect these or objects wrapped in them from insect attack. They may be used in conjunction with other insecticides. Specific compounds are hexachlorotetracyclododecadiene, dodecachlorohexacycloheptadecadiene, dicarbethoxy - hexachlorotetra-cyclododecadiene, octahalotetracyclododecene and heptahalotetracyclododecadiene. Tables are given of the activities of these compared with known insecticides. Specification 614,931 is referred to. The Specification as open to inspection under Sect. 91, includes the subject-matter of Specifications 692,545 and 692,547 and mentions some additional compounds. This subject-matter does not appear in the Specification as accepted.