Stable arylnitrileoxides (1b-e) and 1-ethoxycarbonyl-4-methyl and 6-methyl-1,2-diazepines (2f and 2h) undergo 1,3-cycloaddition reactions to give 1,2,4-oxadiazole derivatives (3) as the most abundant products. Secondary products of the class of isoxazoles (5 or 8) and 4,5-dihydroisoxazoles (6 or 7) were also identified. Overall kinetics were measured at 70.0°C, in mixtures of 1,1,2,2-tetrachloroethane