There is described a new process for the preparation of optically active 2-(2',2'-dihalogenovinyl)-cyclopropane-1-carboxylic acids substituted in the 3 position, and derivatives thereof. A racemate of certain cyclobutanones is reacted with a sulfurous acid salt of an optically active base to obtain a mixture of diastereomeric 4-(2',2',2'-trihalogenoethyl)-cyclobutane-2-sulfonic acid salts; this mixture is then separated into the pure diastereomeric sulfonic acid salts. Either the pure diastereomeric sulfonic acid salts are converted directly to the desired optically active products, or the optically active cyclobutanones obtained from the pure diastereomeric sulfonic acids salts by decomposition are converted, in the presence of a base, to the desired product. Optionally, the product, optically active 2-(2',2'-dihalogenovinyl)-cyclopropane-2-carboxylic acids can be converted to their 1',2'-dibromo derivatives.