Self-assembled monolayers (SAMs) of thiol-porphyrins with different alkyl chain length (abbreviated as H(2)TPPO(CH(2))(n)SH, n = 3, 6, 9 and 12) on gold electrode have been studied by electrochemical impedance spectroscopy in electrolytes containing a one-electron redox couple. From the size and separation of pinholes on SAMs, both estimated the values of charge-transfer resistance and pore size model, we get the surface coverage (theta). The results demonstrate that the surface coverage of saturated porphyrin modified gold electrode is close to unity (> 99%). But on the SAMs, a large number of pinholes still exist and acting as the microelectrodes, with average pinhole radius of 4-6 mu m and the separations are ten times greater than the pinhole size, of 40-70 mu m. Moreover, long chain length thiol-porphyrin SAM shows high surface coverage, larger pinhole size and excellent blocking of electron-transfer reactions between the redox mediator and the gold substrate. We demonstrated the large pinhole size is due to the aggregation of mass individual "micro-holes". (c) 2008 Published by Elsevier B.V.