The exocyclic double bond in 4-arylidene-3-substituted-isoxazolin-5-one (I) undergoes addition reactions with Grignard reagents and with benzene in the presence of aluminiumchloride. Treatment of 4-arylazo derivatives of 3-substituted-isoxazolin-5-ones (III) with Grignard reagents does not effect hetero-ring opening and only the carbonyl group of III enters into reaction, followed by elimination of