Ethyl alkylidenemalonates reacted with sodium azide in dimethyl sulfoxide to give principally triazoles via the first-formed 4,5-dihydro-1H-1,2,3-tiriazoles. The triazolines themselves, and other compounds derived from them, were isolated in trace amounts. Ethyl alkylidenemalonates combined with benzyl azide in a 1,3-dipolar cycloaddition reaction to give 1-benzyl-4,5-dihydro-1H-1,2,3-triazoles. When treated with potassium t- butoxide in dimethyl sulfoxide , these adducts either aromatized to the corresponding triazoles, underwent deethoxycarbonylation but did not aromatize, or lost nitrogen to give ethyl benzylaminomethylidenemalonates. The proclivity for each of these three processes was determined by the C5 substituent of the 4,5-dihydro-1H-1,2,3-triazole. Treatment of the 1-benzyl-4,5-dihydro-1H-1,2,3-triazoles with trifluoroacetic acid gave ethyl diazomalonate.