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Reaction of arene diazonium salts with 3-aminocrotononitrile or methyl 3-aminocrotonate affords the 2-aryl-hydrazono-3-oxobutanenitrile (1or 3and 3′) or the methyl 2-arylhydrazono-3-oxobutanoate (2and 2′ or 4and 4′). A series of these hydrazones has been prepared with a range of electron-withdrawing and -donating substituents in the orthoor paraposition of the aryl moiety. The hydrazones have been characterized by spectroscopic methods, with emphasis on the 1H NMR spectra, which have been used to determine the configuration of the hydrazones as Eor Zor a mixture of the two. The para-substituted hydrazononitriles (1) are formed as a single species, namely the Zisomer, whereas the orthoisomers are formed as a mixture of Eand Zconfigurations (3and 3′). The hydrazonobutanoates (2and 2′ or 4and 4′) are formed as E/Zmixtures regardless of the position of the substituent in the aryl moiety. Complete assignments of all signals in the 1H NMR spectra have been made on the basis of the ability of the various substituents to participate in intramolecular hydrogen bonding and a mechanism is proposed to account for the variations in the proportions of Eand Zisomers and the effect of the nature of the substituent on this ratio. 13C NMR spectra of selected hydrazones have been recorded as an aid to structure assignment. Keywords: hydrazone, diazonium, NMR spectroscopy, E/Zisomers, crotonic acid derivatives.