Compounds of the formula <;FORM:0827435/IV (b)/1>; <;FORM:0827435/IV (b)/2>; and 5,6 - epoxy - 1,2,3,4,7,7 - hexachlorobicyclo-(2,2,1)-2-heptene, are prepared by reacting 1,2,3,4,7,7 - hexachlorobicyclo - (2,2,1) - 2,5-heptadiene with molecular oxygen. Pure oxygen diluted with e.g. nitrogen, or air may be used as a source of oxygen, and the gas is preferably introduced below the surface of the liquid chlorohydrocarbon in a steady stream, preferably through sintered glass or porous materials. Alternatively the oxygen may be passed countercurrently through a moving bed of the starting material. On reacting compound II with hydrochloric acid at elevated temperatures 1,2,3,4,5,5 - hexachloro - 4 - hydroxy methylcyclopent - 2 - ene carboxylic acid chloride is prepared which may be hydrolysed in an alkaline solution to form the corresponding carboxylic acid. The oxidation reaction may be carried out from about 30 DEG C. to about 155 DEG C. at atmospheric to superatmospheric pressures, e.g. up to about 200 lbs. per sq. inch or higher. Catalysts such as ultra-violet light, light from a mercury vapour lamp and chlorine gas may be used to prepare the compounds of the invention. A preferred rate of oxygen influx is at least 100, preferably about 800-2000 c.c. per minute per mol. of 1,2,3,4,7,7 - hexachlorobicyclo - (2,2,1) - 2,5-heptadiene. Solvents may be present in the reaction mixture, especially those which will prevent the reaction temperature rising above 160 DEG C., e.g. xylene. The reaction may be carried out upwards of about two hours. Examples are given under varying reaction conditions.ALSO:The compound <;FORM:0827435/IV (a)/1>; forms a resin on heating and the compound <;FORM:0827435/IV (a)/2>; is used in the preparation of polymers.;FORM:0827435/IV>;FORM:0827435/IV>;FORM:0827435/IV>;FORM:0827435/IV>