Addition of 3,3-dialkyl-, 1,3-dialkyl-, and 1,3,3-trialkyldiaziridines to dibenzoylacetylene, diethyl acetylenedi-oarboxylate, and ethyl propiolate gives, generally, adducts in which the diaziridine ring is no longer intact. For example, addition of 1,3-dialkyl- and 1,3,3-trialkyldiaziridines to dibenzoylacetylene forms 2-(alkylidenehydrazino)-l,4-diphenyl-2-butene-l,4-diones (2). Evidence is presented that it is the alkylated nitrogen of 1-methyl-3,3-pentamethylenediaziridine which adds to the triple bond of dibenzoylacetylene. Stereochemical studies show that diaziridines add to ethyl propiolate to give trans adducts. Hydrolysis of 2-(alkylidenehydrazino)-l,4-diphenyl-2-butene-l,4-diones is shown to be a useful method for the preparation of l-alkyl-3-phenyl-5-benzoylpyrazoles.