Abstract: Isomerization of an unsymmetrical ketoxime ether is effected using at least one Lewis acid selected from titanium chloride, titanium bromide, aluminum chloride, aluminum bromide, boron chloride, and boron bromide as isomerizing agents. Examples of the Lewis acid are titanium tetrachloride, titanium trichloride, aluminum trichloride, aluminum tribromide, boron trichloride, and boron tribromide. The ketoxime ether is preferably an anti-isomer or a syn-isomer of a compound represented by the formula (I) or a mixture of the anti-isomer and the syn-isomer: ##STR1## wherein R.sub.1 represents an aryl group, an aralkyl group or an alkyl group and R.sub.2 and R.sub.3 each represents an aralkyl group or an alkyl group, with a proviso that R.sub.1 and R.sub.2 are not identical.