Abstract: Process for the preparation of 1-lower alkoxycarbonyl-3-(.beta.,.beta.-dichlorovinyl)-2,2-dimethylcyclopropanes from the corresponding 1-cyano-1-carboxylic acid ester by hydrolysis, decarboxylation of the free carboxylic acid, reaction of the cyclopropane nitrile with a lower aliphatic alcohol and anhydrous hydrogen chloride, followed by alcoholysis or hydrolysis of the reaction product so obtained to give the desired ester or carboxylic acid. Includes a method for the separation of essentially pure cis- and trans-isomers from the mixed nitriles. The products are useful intermediates for the preparation of pyrethroid insecticides.