1. The oxidation of some para-substituted benzylamines by diamine oxidase produces the corresponding aldehydes. This was studied to develop a spectrophotometric method for following the enzyme reaction, as the aldehydes produced absorb strongly at 250nm where the substrates are almost optically transparent. 2. p-Dimethylaminomethylbenzylamine was the most useful substrate and full details of its preparation are given. The synthesis of its related oxidation product, p-dimethylaminomethylbenzaldehyde, is also described. 3. The effects of variations in pH, ionic strength, temperature and oxygenation on the reaction are described and the usefulness of the method is illustrated by several applications and assessed by comparison with the standard spectrophotometric assay.
