High-pressure multinuclear NMR probeheads have been developed in order to investigate the effect of pressure on reaction rates. The resulting activation volumes are unique parameters for the assignment of substitution reaction mechanisms. This tool was used to understand the mechanism of chemical and biological systems. For example, solvent exchange at metal ions and optimization of the relaxivity of MRI contrast agents. A high pressure NMR microreactor was built and allowed to monitor the fixation of dinitrogen and dihydrogen on metal ions.