Abstract1The pI50‐values of 1,2‐dimethyl‐, 1,2‐diethyl‐ and 1,2‐dipropyl‐hydrazine are of the order 4,7 for monoamine oxidase (MO) and about 5,8 for diamine oxidase (DO). For both enzymes the pI50‐values of 1,2‐diisopropylhydrazine are much smaller30).2Under conditions similar to those of the enzyme determinations, acetaldehyde reacts non‐enzymically with the first three 1,2‐dialkyl derivatives mentioned above, whilst 1,2‐diisopropylhydrazine remains inert during the spectrophotometric analysis.3In similar systems, 1,2‐dimethylhydrazine reacts slowly with benzaldehyde.4From these observations, and from data obtained in the literature concerning transannular bond formation, it is concluded that 1,2‐dialkylhydrazines react on MO and DO by virtue of their nucleophilic nature. The electrophilic counterpart of the active surface of the enzyme seems to be an aldehyde group in DO and a non‐aldehyde and non‐keton