A comparison of covalent versus non-covalent immobilization of molecular species onto the electrode shows a significant difference in the electron transfer rate. For example, covalently attached pyrene and 1-pyrenylferrocene on indium tin oxide (ITO) yielded pyrene-modified ITO (ITO|Pyr) and ferrocene-modified ITO (ITO|vinylferrocene), respectively. To ITO|Pyr, the redoxactive ferrocene was then physisorbed, resulting in non-covalently attached pyrenylferrocene on ITO|Pyr (ITO|Pyr|pyrenylferrocene, Figure ). A kinetic study on electron transfer was performed on both ITO|Pyr|pyrenylferrocene and ITO|vinylferrocene. The electron transfer rate for the non-covalently modified system, ITO|Pyr|pyrenylferrocene, is about ten times faster than the covalently modified system (kapp = 100 ± 8 s -1 compared to 9 ± 2s -1 ). Although the distance between the ferrocene moiety to the electrode seems shorter in ITO|vinylferrocene, the extent of π-conjugation of the linker was proposed to cause a faster electron transfer in the noncovalent immobilized system. However, opposite trend was found in recent study employing amino-substituted cobalt porphyrin to immobilize on carbon-based electrode. Covalent grafted complex exhibits faster electron transfer rate and higher reactivity comparing to the non-covalently immobilized counterpart.