1-Aryl-2-(1H-imidazol-1-yl)-ethanone oxime ethers are known to exist as a mixture of isomers corresponding to the formulae: ##STR1## To prepare such an isomer in a stereospecifically pure form, the pure stereoisomer of the corresponding ethanone oxime is first converted into an alkali salt in a polar solvent, such as acetone or dimethylformamide, using an alkali in an amount somewhat less than the equimolar amount with respect to the said ethanone oxime, and is then converted to the desired ether by reacting it at a temperature not higher than 40.degree. C. with a halogen compound capable of forming the desired ether. Isolation of the ether product is obtained as the free base or by precipitating it as an acid addition salt upon addition of a suitable organic or mineral acid, preferably nitric acid. A stereospecific ethanone oxime in either its trans- or cis-isomer form corresponding to the formula: ##STR2## can be stereospecifically synthesized from the corresponding 2-halogen ethanone having the formula A--CO--CH.sub.2 --Hal (IV) which, in making the cis oxime (II.sub.c), is first converted with imidazole or a suitably substituted derivative thereof and subsequently oximated by reaction with hydroxylamine in the presence of a molar excess of an alkali at an elevated temperature; or which, in preparing the trans oxime (II.sub.t), is first oximated by reaction with hydroxyl-amine under mild conditions and subsequently converted in the cold with imidazole or a suitably substituted derivative thereof. The products are effective fungicides or bactericides and consequently useful as thermotherapeutic agents in combatting undesirable lower plant organisms in the fields of human as well as veterinary medicine and also as fungicides in agriculture and horticulture.