Online analytics provides insights into the progress of an ongoing reaction without the need for extensive sampling and offline analysis. In this study, we investigated benchtop NMR as an online reaction monitoring tool for complex enzyme cascade reactions. Online NMR was used to monitor a two‐step cascade beginning with an aromatic aldehyde and leading to an aromatic amino alcohol as the final product, applying two different enzymes and a variety of co‐substrates and intermediates. Benchtop NMR enabled the concentration of the reaction components to be detected in buffered systems in the single‐digit mM range without using deuterated solvent. The concentrations determined via NMR were correlated with offline samples analyzed via uHPLC and displayed a good correlation between the two methods. In summary, benchtop NMR proved to be a sensitive, selective and reliable method for online reaction monitoring in (multi‐step) biosynthesis. In future, online analytic systems such as the benchtop NMR devices described might not only enable direct monitoring of the reaction, but may also form the basis for self‐regulation in biocatalytic reactions.

Vicinal amino alcohols such as metaraminol find direct application in pharmaceuticals and serve as building blocks for fine chemicals. The amine transaminase enzyme can facilitate the stereoselective production of such...