Preparation of mixtures of cypermethrin or cyfluthrin isomers enriched in more active species

In the process for the preparation of a mixture of isomers of cypermethrin or cyfluthrin by reacting .alpha.-hydroxy- 3-phenoxyphenylacetonitrile or its 4-fluoro analog with a solution of 3-(2,2-dichloroethenyl)-2,2-dimethylcyclopropanecarbonyl chloride, the improvement that comprises increasing the concentration of the c.sub.2 or t.sub.2 isomers or both relative to the c.sub.1 or t.sub.1 isomers by carrying out the reaction at atmospheric pressure in an inert atmosphere by adding, at a rate such that the reaction does not get out of control, the acetonitrile to a refluxing solution of the acid chloride in a hydrocarbon solvent boiling in the range of about 75 to 115.degree. C., no acid acceptor being present in the reaction mixture, washing the reaction mixture with aqueous base, and recovering the product.