The effect of pressure to several hundred kilobars has been measured on the properties of 13 complexes containing aromatic molecules. These complexes are classified in four groups: those containing tetracyanoethylene (TCNE), those containing iodine, those involving p-phenylenediamine, and halogenated p-benzoquinones, and those involving the halogenated p-benzoquinones and other materials. In all cases the resistance decreased rapidly with increasing pressure at low pressures. A number of complexes exhibited minima in resistance at pressures from 60 to 200 kbar and an irreversible rise at higher pressures. The nature of the irreversible process could in several cases be postulated from difference in the visible and infrared spectra between the transformed and untransformed complexes.