The regio- and stereoselectivity of the cydoadditions of trifluoroacetonitrileoxide (1) with olefins and acetylenes were described. The oxide 1, generated in situ from trifluoroacetohydroximoyl bromide etherate in the presence of triethylamine, reacted with various monosubstituted olefins and acetylenes to give exclusively 5-substituted 3-trifluoromethyl-2-isoxazolines and -isoxazoles, respectively