Ethylenically unsaturated compounds are epoxidized by reaction with an organic peracid in a system containing two incompletely miscible liquid phases, one of which contains the ethylenic compound and the other the organic peracid, the two phases passing in countercurrent to each other. The two phases may pass through the system in simple countercurrent flow but preferably the epoxidation is carried out in a plurality of reaction stages arranged in series so that the feed of ethylenic compound or such part of it as is unreacted passes between successive reaction stages in one direction and the feed of peracid or such part of it as is unreacted passes between said reaction stages in the opposite direction. In such a case it is advantageous to mix the two phases intimately in each reaction stage and then separate the phases before passing countercurrently between the stages. Two to four stages are generally sufficient. A suitable form of apparatus comprising three mixing and separating stages is described. In a preferred embodiment of the process the peracid is formed in situ by adding a peroxide and a carboxylic acid to the last reaction stage. The two phases may comprise an aqueous phase and a non - aqueous phase, the peracid being employed in the aqueous phase. If the ethylenic compound is a water-insoluble liquid, it can constitute the other phase, but it is usually desirable to use a solution of the ethylenic compound in a water-insoluble solvent. Suitable solvents are saturated hydrocarbons, such as n- or iso-pentane, hexanes, octanes or cyclohexane, or aromatic hydrocarbons such as benzene, toluene, xylenes or cumene. Normally, gaseous hydrocarbons, such as ethane, propane or butanes can be used by carrying out the process under sufficient pressure to maintain a liquid phase. Hydrocarbon mixtures, such as fractions of gasoline, kerosine or petroleum may also be used. Halogenated hydrocarbons, such as chloroform, carbon tetrachloride, or chlorine or fluorine substitution products of the above hydrocarbon can be used, as can ethers, higher alcohols and ketones. The process may also be operated with aqueous solutions of water-soluble ethylenic compounds, e.g. allyl alcohol, crotonic acid or mesityl oxide, the peracid being employed in a water-insoluble solvent. To operate under substantially anhydrous conditions, two incompletely miscible liquid organic phases may be used. Thus, unsaturated higher fatty acid esters, e.g. soybean oil, may be epoxidized with an anhydrous methanol solution of peracetic or performic acid in a plurality of stages. The process generally may be carried out using any organic peracid, e.g. peracids derived from fatty acids of one to four carbon atoms, i.e. formic, acetic, chloracetic, propionic, or n- and iso-butyric acids; peracids of higher fatty acids, e.g. caproic, aromatic acids, such as benzoic and toluic acids; or peracids from polycarboxylic acids, such as monoperphthalic or diperphthalic acids, or monoperoxalic or diperoxalic acids, or monopersuccinic or dipersuccinic acids. The process is applicable to the epoxidation of any ethylenic compound, particularly to open chain or cyclic polyethylenic compounds. Suitable ethylenic compounds are 1 : 3-butadiene, pentadienes, cyclopentadiene phenyl butadiene, ethylenic halides, alcohols, ethers, acids, esters, ketones, aldehydes, mercaptans, thioethers, thioacids and esters. Particularly useful starting materials are mono- and diolefinic fusedring compounds having 5 to 6 carbon atoms in each ring, e.g. 1 : 4-methano-2 : 5-cyclo-hexadiene, hexa- and octa-hydronaphthalenes, 2 : 3-dichloro - 1 : 4 - methano - 2 : 5 - cyclohexadiene, 2 : 3 - dichloro - 1 : 4 : 4a : 5 : 8 : 8a-hexahydro - 1 : 4 : 5 : 8 - exo - endo - dimethanonaphthalene and 5 : 6 - dichloro - 4 : 7 - methano - 3a - 4 - 7 - 7a - tetrahydroindene. Epoxidations described in Specification 692,547 are carried out with special advantage. In general, temperatures below 100 DEG C. are preferred, 0 DEG to 75 DEG C. being suitable. Reaction times vary from 30 minutes to 48 hours. The pressure may be atmospheric, higher or lower but must be sufficient to maintain the two liquid phases. When forming the peracid in the final stage, in general, between 0.75 and 2 mols. of peroxide are used per mol. of ethylene unsaturation in the starting material. The peroxide concentration in the aquous phase of the final stage is preferably below 35 per cent and the carboxylic acid content below 60 per cent. Hydrogen peroxide of 25-80 per cent concentration may be used with formic or acetic acid in a overall ratio of 0.5 to 2 mols. per mol. of ethylenic compound. An acid catalyst such as sulphuric acid or phosphoric acid may be used. Detailed examples are given of the expoxidation of (1) a benzene solution of aldrin to dieldrin in three stages using peracetic acid formed in situ in the presence of sulphuric acid; (2) a benzene solution of aldrin to dieldrin in two stages using hydrogen peroxide, acetic acid and sulphuric acid; (3) soybean oil in benzene solution as in (2) with performic or peracetic acid.