A novel thiol-derivative porphyrin (5-[p-(mercaptopropyloxy)-phenyl]-10,15,20-triphenylporphine shortly as H2MPTPP) and its cobalt(II) complex (NVTPPCo(II)) have been successfully immobilized on gold surface by self-assembly technique. The surface coverages (F) of MPTPPCo(II) was calculated to be 4.86 x 10(-11) mol/cm(2) by investigating the reductive desorption of S-Au which demonstrated that the monolayer was not packed well and had many interval pinholes. The gold electrodes modified with MPTPPCo(II) and MPTPPCo(II)+thiolglycolic acid show good activity to the two-electron catalytic reduction of O-2 to H2O2. Nanoscopic structures gained by atomic force microscopy (AFM) not only provided further evidence for the successful formation and immobilization of such monolayers, but also elucidated the potential reasons for the electrocatalytic activity of such monolayers to dissolved oxygen, which was proved to be very interesting and significant that the mixed monolayer could enhance the electroreduction and proceed more readily for the electroreduction of O-2. (c) 2005 Elsevier B.V. All rights reserved.