The design of sample flow cells, commonly used in on-line analytics and especially for medium resolution NMR spectroscopy (MR-NMR) in low magnetic fields, was experimentally and theoretically investigated by H-1 NMR and numerical simulations. The flow pattern was characterised to gain information about the residence time distribution and mixing effects. Both H-1 NMR imaging and spectroscopy were used to determine the characteristics of flow cells and their significance for on-line measurements such as reaction monitoring or hyphenated separation spectroscopy. The volume flow rates investigated were in the range from 0.1 to 10 ml/min, typically applied in the above mentioned applications. The special characteristics of flow cells for MR-NMR were revealed by various NMR experiments and compared with CFD simulations and to flow cells commonly used in high-field NMR. The influence of the design of the inlet and outlet on the flow pattern was investigated as well as the effect of the length of the cell. For practical use, a numerical estimation of the inflow length was given. In addition, it was shown how experiments on the polarisation build-up revealed insight into the flow characteristics in MR-NMR. (C) 2012 Elsevier Ltd. All rights reserved.