Aromatic hydrazides of the general formula NH2NHCO(CH2)nC6H4R were covalently bound by bovine serum amine oxidase (BSAO), giving rise to optical and CD absorptions at 350-400 nm. Benzohydrazides (n = 0) reacted slowly, in the ratio of one per dimeric protein molecule, like semicarbazide. Phenylacetohydrazides (n = 1) and phenylpropionic hydrazides (n = 2) reacted instead in the ratio of two per dimer, one molecule at a much faster rate than the other. The fast reaction correlated with the loss of enzymatic activity. The contribution to the optical absorbance of either molecule was identical, but only the first one produced a CD band, the wavelength and sign of which were determined by the number n of methylene groups in the hydrazide. In n = 1 and n = 2 compounds, the reaction was faster as the R substituent became more hydrophobic (triazolyl less than imidazolyl less than phenyl), suggesting a specific interaction with the protein matrix. Phenylhydrazine was found to react with the native enzyme in the ratio of only one per protein dimer. However, one phenylhydrazine was also slowly bound by most 1:1 enzyme-hydrazide adducts, with the formation of ternary derivatives. Phenylhydrazine formed the usual intense band at 447 nm with n = 1 and n = 2 hydrazide-BSAO adducts and a weaker, blue-shifted band with the adducts of semicarbazide and of some n = 0 hydrazides. In both cases, the hydrazide absorption band was unaffected. Competition was observed with other benzohydrazides and with the second molecule of n = 1 compounds. A half-site mechanism appears to be operative, the second site being always less reactive than the first. Reactivity and adduct conformation were also affected by N,N-diethyldithiocarbamate, a powerful enzyme inhibitor that binds copper.