This thesis investigates the opportunities of process control using low-field NMR (nuclear magnetic resonance) instruments that are equipped with permanent magnets. These low-field NMR instruments are able to operate stably in industrial environments because they are less prone to environmental effects than their superconductive counterparts. The impact of prominent environmental factors such as ambient temperature and surrounding magnetic fields are investigated. The optimization of the on-line sampling process includes a reduction in measurement delays due to optimization of the tubing. A study on the effect of flow cell design and volume is part of this thesis. With respect to the obtained knowledge an esterification of acetic acid and ethanol as well as a Knoevenagel condensation of benzaldehyde and dimethyl malonate are chosen as model reaction and investigated at different starting conditions. Due to online monitoring of the reaction the impact of changes in starting material on the reaction yield as well as the reaction rate can be followed and anomalies can be spotted in real time.