This dissertation aims to establish experimental methods to characterize transient states of chemical reactions by stabilizing transient states using mechanical stress. We argue that gigapascal (GPa)-level pressure produces deformed molecular structures that approximate the transient states along the reaction coordinate on the ground-state surface, thereby fundamentally understanding their mechanism through high-pressure mechanochemistry. By varying the pressure, the entire reaction path can be quasi-statically traversed and characterized with steady-state measurements. This strategy is experimentally implemented using a diamond anvil cell and in situ spectroscopy and highlighted in experimentation with three model systems. Two models investigate dicopper peroxo-centered isomerizations which are vastly found in nature within coupled binuclear copper centers. The cis-, trans-, and side-on peroxo and bis-(µ -oxo) isomers each play a unique and crucial role in the type (III) copper catalytic cycles. Through thes