A simple device for taking in situ proton NMR measurements in (H2O)-H-1 is described. This allows aeration of reactions in a 10 mm diameter NMR tube without modifying the magnet or the probe head. With this device, aerobic biotransformations can be monitored in the NMR-tube placed in the spectrometer. It allows in situ analyses of the transformations, separating the aeration period temporally from the measurement time, not unlike traditional Warburg respiratory experiments. Two reactions determining kinetic and stoichieometric parameters: (i) a biotransformation by a growing Pseudomonas putida culture and (ii) L-phenylalanine oxidation catalysed by L-amino acid oxidase [E.C. 1.4.3.2]; both incubations were contained in the magnet.